To investigate the possibility that there might be an increased risk of heterologous invasive bacterial disease after routine childhood immunization with measles, mumps, and rubella vaccine live; diphtheria and tetanus toxoids and pertussis vaccine; and oral poliovirus vaccine live, a case-control study was conducted within the Kaiser Permanente Northern California pediatric population. Contrary to the premise, an apparent protective effect against invasive bacterial disease was detected after all childhood vaccinations. However, when adjustment was made for frequency of well-care visits and day-care attendance, no significant relationship was seen between receipt of routine childhood immunizations and risk of invasive heterologous bacterial disease for any individual vaccine, although a statistically significant protective effect was detected within 1 or 3 months after the receipt of any vaccine. Since a decreased risk of invasive bacterial disease was also noted to be related to the receipt of routine well-child pediatric care, other preventive health care measures may be responsible for the apparent immunization protective effect.