AIMS--To examine the relation between immunisation and the risk of sudden infant death syndrome (SIDS). METHODS--A large nationwide case-control study. Parental held records were used to measure immunisation status. RESULTS--Infants were at increased risk of SIDS if they had not received the 6 week, 3 month, and 5 month immunisations. After controlling for potential confounding variables, including those which measured health care use and infant illness, the relative risk of SIDS for infants not being immunised at 6 weeks was 2.1 (95% confidence interval = 1.2, 3.5). Four percent of cases died within four days of immunisation and 7.6% of control infants had been immunised within four days of the nominated date. There was a reduced chance of SIDS in the four days immediately following immunisation (OR = 0.5; 95% CI = 0.2 to 0.9). CONCLUSIONS--Immunisation does not increase the risk of SIDS and may even lower the risk.