Abstract New Zealand's high mortality rate from sudden infant death syndrome (SIDS) prompted the development of the New Zealand Cot Death Study. A report of the analysis of the data from the first year has been published. This report now gives the major identified risk factors from the full 3 year data set. In this case-control study there were 485 infants who died from SIDS in the post-neonatal age group, and 1800 control infants, who were a representative sample of all hospital births in the study region. Obstetric records were examined and parental interviews were completed in 97.5% and 86.9% of subjects, respectively. As expected many risk factors for SIDS were confirmed including: lower socio-economic status, unmarried mother, young mother, younger school-leaving age of mother, younger age of mother at first pregnancy, late attendance at antenatal clinic, non-attendance at antenatal classes, Maori, greater number of previous pregnancies, the further south the domicile, winter, low birthweight, short gestation, male infant and admission to a special care baby unit. In addition, however, we identified four risk factors that are potentially amenable to modification. These were the prone sleeping position of the baby (odds ratio (OR) = 3.70; 95% confidence interval (Cl) = 2.91, 4.70); bed sharing (OR = 2.70; 95% Cl = 2.02, 3.62); maternal smoking of 1–9 cigarettes per day (OR = 3.47; 95% Cl = 2.50, 4.83), 10–19 cigarettes per day (OR = 3.94; 95% Cl = 2.87, 5.41) or more than 20 cigarettes per day (OR = 5.90; 95% Cl = 4.20, 8.31); and not breast feeding (OR = 2.39; 95% Cl = 1.88, 3.04). After controlling for all of the above variables, the relative risks associated with prone sleeping position (OR = 4.84). sharing bed (OR = 2.02), maternal smoking (OR = 1.79) and not breast feeding (OR = 1.89) were still statistically significant. Population-attributable risk calculations suggest that these four risk factors may account for 82% of deaths from SIDS. The SIDS mortality rate may fall to less than 0.7/1000 live births if all parents stop putting their infants down to sleep in the prone position, do not sleep with their baby, do not smoke, and breast feed their infants.