To evaluate recent immunization against diphtheria, tetanus, and pertussis (DTP) as a possible risk factor for sudden infant death syndrome (SIDS), we studied the rates of SIDS after the administration of DTP vaccine in a cohort of 129,834 children who were born in four urban Tennessee counties during the period from 1974 through 1984. All the children received at least one DTP immunization in the first year of life at county health-department clinics or from Medicaid providers. Computerized immunization records from these sources were linked with Tennessee birth and death certificates to establish the cohort, ascertain the timing of immunization, and identify cases of SIDS. These children represented 42 percent of the births in the four counties. Among these children, 204 deaths occurred at the ages of 29 to 365 days; 109 deaths were classified as due to SIDS. We estimated the risk of SIDS according to the length of time, up to 30 days, since DTP immunization and compared it with the risk 31 days or more after immunization to calculate the relative risk. With control for age, the relative risk from 0 to 3 days after DTP immunization was 0.18 (95 percent confidence interval, 0.04 to 0.8); from 4 to 7 days, 0.17 (95 percent confidence interval, 0.04 to 0.7); from 8 to 14 days, 0.75 (95 percent confidence interval, 0.4 to 1.5); and from 15 to 30 days, 1.0 (95 percent confidence interval, 0.6 to 1.6). A multivariate analysis in which we controlled for age, sex, race, year, birth weight, and Medicaid enrollment, produced similar results. We conclude that in this large population of children there was no increase in the risk of SIDS after immunization with the DTP vaccine.