The possible association between diphtheria-pertussis-tetanus (DTP) immunization and the subsequent occurrence of sudden infant death has been examined using data from the National Institute of Child Health and Human Development (NICHD) Sudden Infant Death Syndrome (SIDS) Cooperative Epidemiological Study, a large multicenter, population-based, case-control study. In a preliminary report based on the first 400 eligible singleton SIDS victims and 800 matched living control infants, no temporal association between SIDS and DTP immunization was found. From the final sample of 800 eligible singleton SIDS victims, 95% (n = 757) were defined as definitely or probably having died of SIDS on the basis of pathology data. Data from these 757 case infants and their corresponding control infants (n = 1,514) are presented in this report. Two control infants, both living, were randomly selected for each case infant: an age-matched control A and an age-, race-, and low birth weight-matched control B. Overall, case infants were less likely to have received any DTP immunization. Only 39.8% of case infants had received at least one DTP immunization compared to 55.0% of control A infants and 53.2% of control B infants. Based on maternal interviews and postnatal medical records, 1.8% of case infants (five infants) immunized with DTP died within the first 24 hours following immunization. Similarly, 5.0% of control A infants (n = 21) and 2.2% of control B infants (n = 9) had been immunized within 24 hours of the maternal interview, which represents the comparable time frame for the age-matched control infants. These results confirm the earlier preliminary findings from the NICHD SIDS Cooperative Epidemiological Study and suggest that DTP immunization is not a significant factor in the occurrence of SIDS.