Objective. To evaluate the relative frequency of adverse reactions after initial and subsequent immunizations among infants receiving primary immunization with acellular (DTaP) or whole-cell (DTP) pertussis vaccine with diphtheria and tetanus combined. Methods. We examined the occurrence of common reactions in 2127 infants within 48 hours after immunization at 2, 4, and 6 months with one of 13 DTaP or with Lederle DTP (WCL). Data on at least two consecutive immunizations were available for 357 WCL recipients and 1770 DTaP recipients. For these analyses, reactions evaluated included fever of 100.4°F (38°C) or greater, redness of 21 mm or larger, swelling of 21 mm or larger, moderate or severe pain, moderate or severe fussiness, loss of appetite, drowsiness, and vomiting. Results. With one exception, reactions were approximately 1.5 to 8 times more likely to occur in WCL recipients if the same reaction had been observed at the previous immunization (the single exception was redness after the second immunization). Both initial and repeated reactions were less likely in DTaP than in WCL recipients. As with WCL recipients, risks of repeated reactions in DTaP recipients were higher than the risks of initial reactions (from 2.5 to 24 times as high). Conclusion. Reactions after a second or third immunization with either WCL or DTaP vaccine are more likely to occur in infants who had the same reaction after the preceding immunization. Absolute risks of repeated reactions tended to be lower after DTaP vaccine than after the WCL vaccine.