This retrospective study (1979 to 1986) investigated the possible etiologic relationship between vaccination and aseptic meningitis in 115 hospitalized children who became ill within 30 days of vaccination with the Leningrad 3 strain of mumps virus and the Edmonston-Zagreb strain of measles virus. The etiologic viral diagnosis was based on serologic tests and the isolation of virus from cell cultures which distinguished between attenuated and "virulent" mumps virus. The incidence of mumps vaccine-associated meningitis was 1/1000 vaccine recipients. In 92% of children the incubation period was 11 to 25 days and 28% had associated swelling of the salivary glands. Sixteen cases (13.9%) had a positive cerebrospinal fluid culture (attenuated mumps virus, 6 cases; "virulent" mumps virus, 7 cases; echoviruses, 3 cases). Clustering of cases, seasonal occurrence and age of the patients suggested causal relationship with the vaccination in the majority of children. In 4 patients with attenuated virus isolation from cerebrospinal fluid the incubation period ranged from 17 to 20 days. Clinical findings did not differ from natural mumps meningitis. The course was uncomplicated and at discharge the patients had no sequelae. Measles virus was never found as a cause of the meningitis. The mumps vaccine virus should be recognized as one of the causative agents of aseptic meningitis in countries where less attenuated mumps vaccine is used.