Background:Results of case-control studies in the past 5 years suggest that the epidemiology of sudden infant death syndrome(SIDS)has changed since the 1991 UK Back to Sleep campaign.The campaign’s advice that parents put babies on their back to sleep led to a fall in death rates.We used a longitudinal dataset to assess these potential changes.Methods:Population-based data from home visits have been collected for 369 consecutive unexpected infant deaths(300 SIDS and 69 explained deaths)in Avon over 20 years(1984-2003).Data obtained between 1993 and 1996 from 1300 controls with a chosen “reference”sleep before interview have been used for comparison.Findings:Over the past 20 years,the proportion of children who died from SIDS while co-sleeping with their parents,has risen from 12%to 50%(p < 0.0001),but the actual number of SIDS deaths in the parental bed has halved(P=0.01).The proportion seems to have increased partly because the Back to Sleep campaign led to fewer deaths in infants sleeping alone-rather than because of a rise in deaths of infants who bed-shared,and partly because of an increase in the number of deaths in infants sleeping with their parents on a sofa.The proportion of deaths in families from deprived socioeconomic backgrounds has risen from 47%to 74%(P=0.003),the prevalence of maternal smoking during pregnancy from 57%to 86%(P=0.0004),and the proportion of pre-term infants from 12%to 34%(P=0.0001).Although many SIDS infants come from large families,first-born infants are now the largest group.The age of infants who bed-share is significantly smaller than that before the campaign,and fewer are breastfed.Interpretation:Factors that contribute to SIDS have changed in their importance over the past 20 years.Although the reasons for the rise in deaths when a parent sleeps with their infant on a sofa are still unclear,we strongly recommend that parents avoid this sleeping environment.Most SIDS deaths now occur in deprived families.To better understand contributory factors and plan preventive measures we need control data from similarly deprived families,and particularly,infant sleep environments. Background: Results of case-control studies in the past 5 years suggest that the epidemiology of sudden infant death syndrome (SIDS) has changed since the 1991 UK Back to Sleep campaign. The campaign’s advice that parents put babies on their back to sleep led to a fall in death rates. We used a longitudinal dataset to assess these potential changes. Methods: Population-based data from home visits have been collected for 369 consecutive unexpected infant deaths (300 SIDS and 69 explained deaths) in Avon over 20 years (1984 -2003). Data obtained between 1993 and 1996 from 1300 controls with a chosen “reference” sleep before interview have been used for comparison. Findings: Over the past 20 years, the proportion of children who died from SIDS while co-sleeping with their parents, has risen from 12% to 50% (p <0.0001), but the actual number of SIDS deaths in the parental bed has halved (P = 0.01). fewer deaths in infants sleeping alone-rather than because of a rise in deaths of infants who bed-shared, and partly because of an increase in the number of deaths in infants sleeping with their parents on a sofa.The proportion of deaths in families from deprived socioeconomic backgrounds has The prevalence of maternal smoking during pregnancy from 57% to 86% (P = 0.0004), and the proportion of pre-term infants from 12% to 34% (P = 0.0001) Although many SIDS infants come from large families, first-born infants are now the largest group. The age of infants who bed-share is significantly smaller than that before the campaign, and fewer are breastfed. Premise: Factors that contribute to SIDS have changed in their importance over the past 20 years.Although the reasons for the rise in deaths when a parent sleeps with their infant on a sofa are still unclear, we strongly recommend that parents avoid this sleeping environment. Most SIDS deaths now occur in deprived families.To better understand contributory factors and plan preventive measures we need control data fromitable deprived families, and particularly, infant sleep environments.