BACKGROUND: According to the Israeli immunization schedule, 1 year old babies should receive two concomitant vaccinations: MMR (measles-mumps-rubella), and DTap-Hib-IPV (diphtheria tetanus acellular pertussis-Haemophilus influenzae type b-poliomyelitis). However, about one-third of infants in Israel receive these vaccinations separately. Nurses at a primary care prevention clinic in Israel observed that the separate mode of vaccination is associated with a lower rate of side effects. OBJECTIVES: To validate this observation and determine whether it represents an exception or the rule. METHODS: A nested prospective follow-up study was conducted in a primary care clinic in Israel. The survey included 191 mothers and their offspring born during 2004/2005. The mothers were interviewed over the telephone 2 weeks after the day of vaccination. RESULTS: The rate of adverse effects in children who received the injections separately was significantly lower than among those who were vaccinated simultaneously (40% vs. 57%). CONCLUSIONS: It may be necessary to reconsider the current vaccination policy regarding concomitant injections.