BACKGROUND: In New Zealand, an association has been shown between postnatal depression and sudden infant death syndrome (SIDS). AIM: To replicate the New Zealand study. DESIGN OF STUDY: Case-control study. SETTING: The city of Sheffield, UK. METHOD: The database of the Sheffield Child Development Study was used Demographic and obstetric data were collected and at one month postpartum the Edinburgh Postnatal Depression Scale (EPDS) was administered. Detailed information on the cause of all infant deaths was available. RESULTS: There were 32,984 live births during the study period (from the year 1988 to 1993) and 42 babies died with the cause registered as SIDS. Multivariate analysis showed that smoking was the most important risk factorfor SIDS (odds ratio [OR] = 7.24, 95% confidence interval [95% CI] = 2.76 to 19.01), followed by a high EPDS (OR = 3.20, 95% CI = 1.46 to 6.99) and residence in an area of poverty (OR = 2.33, 95% CI = 1.06 to 5.11). CONCLUSIONS: The Sheffield data confirm the New Zealand findings. A high EPDS score and, by implication, postnatal depression, may be risk factors for SIDS, however, there are many possible explanations for the association.