Objective. Severe abnormalities of the head ultrasound (HUS)-are important predictors of cerebral palsy (CP) and mental retardation, and a normal HUS usually ensures the absence of major impairments. With the increasing survival of extremely low birth weight (ELBW) infants (birth weight < 1000 g), the prognostic significance of a normal HUS may differ. This study examined the prevalence of and risk factors for CP and impaired mental development among ELBW infants with a normal HUS. Methods. Study infants were ELBW infants who were cared for in Neonatal Research Network centers in the years 1995-1999, had a normal early and late HUS, survived to discharge, and returned for follow-up assessments at 18 to 22 months’corrected age. The outcomes of interest were a score < 70 on the Bayley Scales of Infant Development-II Mental Developmental Index (MDI) and CP. Risk factors included maternal demographics; infant characteristics; and interventions or morbidities related to the lung, infection, and nutrition. Logistic regression was used to estimate odds ratios (ORs) and 95%confidence intervals (CIs). A time-oriented approach was used to select variables for inclusion in logistic models. Results. Of 1749 infants with a normal early and late HUS (performed at a mean age of 6 and 47 days, respectively), 1473 (84%) returned for follow-up assessment. Infants had a birth weight of 792 ±134 g (mean ±SD) and gestational age of 26 ±2 weeks. Rates of CP and MDI < 70 were 9.4%and 25.3%, respectively, and 29.2%of infants had either CP or MDI < 70. In multivariate analyses, factors associated with CP were male gender (OR: 1.8; 95%CI: 1.2-2.6),multiple birth, (OR: 1.6; 95%CI: 1.1-2.5), decreasing birth weight (OR: 1.3 for each 100-g decrease; 95%CI: 1.1-1.5), pneumothorax (OR: 2.3; 95%CI: 1.2-4.4), and days of conventional ventilation (OR: 1.2 for each additional 10 days; 95%CI: 1.1-1.3). With the exception of pneumothorax, these same factors were associated with MDI < 70, in addition to less maternal education (OR: 1.4; 95%CI: 1.0-1.9) and Medicaid or lack of coverage for maternal insurance (OR: 1.7; 95%CI: 1.2-2.4). Conclusions. Nearly 30%of ELBW infants with a normal HUS had either CP or a low MDI. Risk factors that are associated with this high rate of adverse outcomes include pneumothorax, prolonged exposure to mechanical ventilation, and educational and economic disadvantage. Improvements in pulmonary care to reduce duration of ventilation and avoid air leaks might improve neurodevelopmental outcome for ELBW infants. Objective. Severe abnormalities of the head ultrasound (HUS) -are important predictors of cerebral palsy (CP) and mental retardation, and a normal HUS usually ensures the absence of major impairments. With the increasing survival of extremely low birth weight (ELBW) infants (birth weight <1000 g), the prognostic significance of a normal HUS may differ. This study examined the prevalence of and risk factors for CP and impaired mental development among ELBW infants with a normal HUS. Methods. Study infants were ELBW infants who were cared for in Neonatal Research Network centers in the years 1995-1999, had a normal early and late HUS, survived to discharge, and returned for follow-up assessments at 18 to 22 months’corrected age. The outcomes of interest were a score < 70 on the Bayley Scales of Infant Development-II Mental Developmental Index (MDI) and CP. Risk factors included maternal demographics; infant characteristics; and interventions or morbidities related to the lung, infection, and nutrition. Logistic regression was used to estimate odds ratios (ORs) and 95% confidence intervals (CIs). A time-oriented approach was used to select variables for inclusion in logistic models. Results. Of 1749 infants with a normal early and late Hants (performed at a mean age of 6 and 47 days, respectively), 1473 (84%) returned for follow-up assessment. Infants had a birth weight of 792 ± 134 g (mean ± SD) and gestational age of 26 ± 2 Rates of CP and MDI <70 were 9.4% and 25.3%, respectively, and 29.2% of infants had either CP or MDI <70. In multivariate analyzes, factors associated with CP were male gender (OR: 1.8; 95% CI : 1.2-2.6), multiple birth (OR: 1.6; 95% CI: 1.1-2.5), decreasing birth weight (OR: 1.3 for each 100 -g decrease; 95% CI: 1.1-1.5), pneumothorax 2.3; 95% CI: 1.2-4.4), and days of conventional ventilation (OR: 1.2 for each additional 10 days; 95% CI: 1.1-1.3). With the exception of pneumothorax, these same factors were associated with MDI <70 in addition toless maternal education (OR: 1.4; 95% CI: 1.0-1.9) and Medicaid or lack of coverage for maternal insurance (OR: 1.7; 95% CI: 1.2-2.4). Conclusions. Nearly 30% of ELBW infants with a normal Hus had either CP or a low MDI. Risk factors that are associated with this high rate of adverse outcomes include pneumothorax, prolonged exposure to mechanical ventilation, and educational and economic disadvantages. Improvements in pulmonary care to reduce duration of ventilation and avoid air leaks might improve neurodevelopmental outcome for ELBW infants.