To provide accurate information on the common sequelae of measles-mumps-rubella (MMR) vaccination and to compare post-vaccine symptoms in children vaccinated at 13 and 15 months.Prospective cluster randomized controlled trial.Twenty-two family practices in southwestern Ontario.All 376 infants who were due to receive MMR vaccine in the next year, 253 (67.3%) successfully completed the study.MMR vaccine administered at 13 months by half of the family physicians and at 15 months by the remaining half.Family physician's physical findings in children 7 days and 30 days after vaccine; reported illnesses by mothers in a daily diary in the month before and after vaccination and medical records of visits to family physicians and hospital admissions in the month before and after vaccination.Compared with the incidence rates in the corresponding weeks before vaccination, the rates of lymphadenopathy (23.8%) and fever (16.8%) were higher 1 week afterward and the rate of rash (26.9%) was higher 7 to 14 days afterward. Fewer health problems were reported in the third and fourth weeks after vaccination than in the corresponding weeks beforehand. Hospital admissions after vaccination were no more frequent than those before once cause and time of admission were taken into account. The two age groups did not differ in any of the outcomes.Mothers should be informed about the possibility of increased physical findings in the weeks after MMR vaccination, especially lymphadenopathy, nasal discharge and rash. Since the occurrence of sequelae does not seem to differ significantly between 13-month-old recipients and 15-month-old recipients, it should not influence the decision of when to administer the vaccine.