Background: Several adverse events following immunization (AEFI) have been attributed to immunization with live attenuated measles, mumps, and rubella (MMR) vaccines. The MMR vaccine was introduced into the routine infant immunization schedule in 2003, followed by a second dose of vaccine at school-entry for children 4 to 6 years of age. The objective of this study was to characterize adverse reactions following MMR vaccination in Iran.Methods: Children who received the MMR vaccine and resided in five selected provinces of Iran were examined weekly for four weeks to detect well-known AEFIs that included: parotitis, fever and convulsions, convulsions without fever, encephalopathy, and anaphylactic reactions. Incidence of AEFIs were calculated and compared among recipients in both age groups.Results: During the follow-up period, trained providers reported 792 AEFIs. Parotitis was the most frequent event occurring in 1.8% of recipients. Of 14, 109 children vaccinated at 12 months of age the following AEFIs occurred: parotitis (147), fever and convulsions (8), convulsions (7), encephalopathy (1), and anaphylactic reactions (1) . Of 29, 338 children vaccinated at 4 to 6 years of age, parotitis, fever and convulsions, encephalopathy, and anaphylaxis occurred in 626, 5, 1, and 1 child, respectively; no convulsions without fever were reported in this age group.Conclusion: Parotitis is the most frequent AEFI among MMR vaccine recipients in Iran. Incidence rates of AEFIs following MMR vaccination in Iran are similar to rates of AEFIs reported in other studies.