Late-onset hospital-acquired sepsis is common in extremely low birth-weight ( < 1000g) (ELBW) infants. The diagnosis is difficult since, at early stages of sepsis, routine laboratory tests are neither specific nor sensitive. In term infants with sepsis neutrophil surface expression of CD11b/CD18, a β 2-integrin, is significantly increased. Here we studied whether increased CD11b/CD18 density on blood neutrophils and monocytes serves as an early sepsis marker in ELBW infants. Blood samples were obtained from 30 ELBW infants on a daily basis for 3-4 postnatal weeks, and neutrophil and monocyte CD11b/CD18 expression was determined by flow-cytometry. Patients were assigned one of 3 groups: 1) an infected group, comprised of infants who had blood culture-positive sepsis and/or necrotizing enterocolitis, 2) a non-infected group, and 3) a potentially infected group, comprised of infants in whom infection was suspected but could not be confirmed microbiologically. One patient had blood culture contamination and was excluded from the analysis. In the infected group, CD11b expression gradually increased during the three days preceding sampling for blood culture. At the day of sampling, median expression of CD11b in neutrophils and monocytes was higher in the infected group than in the control group. For neutrophils the sensitivity and specificity were 1.00 and 0.56, respectively, and for monocytes, 0.86 and 0.94, respectively. From these data, we conclude that determination of CD11b/CD18 density on neutrophils and monocytes may improve diagnosis of late-onset sepsis in ELBW infants. Late-onset hospital-acquired sepsis is common in extremely low birth-weight (<1000g) (ELBW) infants. The diagnosis is difficult since, at early stages of sepsis, routine laboratory tests are neither specific nor sensitive. In term infants with sepsis Neutrophil surface expression of CD11b / CD18, a β 2-integrin, is significantly increased. Here we studied whether increased CD11b / CD18 density on blood neutrophils and monocytes serves as an early sepsis marker in ELBW infants. Blood samples were obtained from 30 ELBW infants on a daily basis for 3-4 postnatal weeks, and neutrophil and monocyte CD11b / CD18 expression was determined by flow-cytometry. Patients were assigned one of 3 groups: 1) an infected group, comprised of infants who had blood culture-positive sepsis and / or necrotizing enterocolitis, 2) a non-infected group, and 3) a potentially infected group, comprised of infants in whom infection was suspected but could not be confirmed microbiologically. One patient had blood cultur In the infected group, CD11b expression gradually increased during the three days preceding sampling for blood culture. At the day of sampling, median expression of CD11b in neutrophils and monocytes was higher in the infected group than in the control group. For neutrophils the sensitivity and specificity were 1.00 and 0.56, respectively, and for monocytes, 0.86 and 0.94, respectively. From these data, we conclude that determination of CD11b / CD18 density on neutrophils and monocytes may improve diagnosis of late- onset sepsis in ELBW infants.