We evaluated the effect of infant vaccination with HbOC Haemophilus influenzae type b (Hib) conjugate vaccine on the risk of onset of type 1 juvenile diabetes later in life by examining data from a large controlled prospective Phase III clinical efficacy trial conducted within Northern California Kaiser Permanente between 1988 and 1990. The overall study population included children who were offered the Hib conjugate vaccine (acceptors and refusers) as well as a cohort of children who were systemically excluded from the trial on the basis of their birth date. These children are now 10 to 12 years of age. We found no evidence that vaccination with Hib conjugate vaccine in infancy is associated with risk of diabetes later in life.