The b-CAPSA I capsular polysaccharide vaccine for Haemophilus influenzae type b was given to 87,541 children 2 through 5 years of age in the Kaiser Permanente Medical Care Program, and the children were then followed using a multiple modality surveillance. Phase 1 consisted of 24-hour recall of immediate side effects which were recorded on questionnaires given to families of 13,500 children. Local side effects were found to be uncommon: 2.3% had a temperature of greater than or equal to 38.3 degrees C (greater than or equal to 101 degrees F); 4.8% had local erythema, 2.9% local swelling, and 12.6% local tenderness; two children had wheezing shortly after immunization. In Phase 2, 30 days after immunization, questionnaires were mailed to parents of all 87,541 children, who were asked to respond to questions about illnesses and health care. Phase 3 consisted of active surveillance of patient health care use by physicians and nurses during the 30 days after immunization. During the 30-day reporting periods, there were 40 hospitalizations, including one for wheezing and one for febrile seizure. Of the 40 hospitalizations, only the one for wheezing was believed by the admitting physician to be probably associated with vaccine administration. Three children had seizures within 30 days of immunization. None of the seizures was believed by the reporting physician to be associated with immunization. Adverse effects of the vaccine were mild, limited to local reactions and occasional temperature elevation; bronchospasm after immunization occurred rarely.