AbstractThis study was carried out in response to reports from nurses to a post‐neonatal mortality review committee that a number of mothers of infants dying from sudden infant death syndrome (SIDS) appeared to be depressed before the child's death. The New Zealand Cot Death Study was a 3 year multicentre case‐control study for SIDS. There were 485 SIDS cases in the post‐neonatal age group in the study regions, and these were compared with 1800 control infants. Infants of mothers with either a self‐reported use of medication for psychiatric disorders, a history of hospitalization for psychiatric illness or a family history of postnatal depression had a significantly increased risk of SIDS compared with infants of mothers who were either not using medication (odds ratio (OR) = 1.45; 95% confidence interval (Cl) = 1.03, 2.04) or were without a history of hospitalization for psychiatric illness (OR = 1.80; 95% Cl = 1.03, 3.11) or a family history of postnatal depression (OR = 1.61; 95% Cl = 1.06, 2.43). All mothers of infants born in the study areas over a 1 year period were eligible to complete a questionnaire measuring maternal depression when the infant was 4 weeks of age. Thirty‐three infants subsequently died from SIDS, and they were compared with 174 controls. Fifteen (45.5%) of the mothers of cases were depressed, compared with 28 (16.1%) of the mothers of controls. This prospective study found that the infants of those mothers that were depressed were more likely to die from SIDS than those of the non‐depressed mothers (OR = 4.35; 95% Cl = 1.82, 10.37) and postnatal depression as a risk factor for SIDS was still significant after controlling for possible confounding variables (OR = 3.37; 95% Cl = 1.24, 9.12). We conclude that postnatal depression is a risk fact