To study the outcome of mumps virus encephalitis 47 patients were contacted 1-15 years after the acute encephalitis associated with mumps virus infection. Twenty-three patients experienced clinical sequelae such as difficulties in memory and learning, focal motor or sensory signs, and loss of hearing and visual acuity. Lumbar puncture was performed on 8 patients. Antibodies to mumps virus were detected in 6 cerebrospinal fluid (CSF) specimens using enzyme immunoassay and in 3 patients an abnormal serum/CSF antibody ratio was observed 11, 26 and 58 (controls greater than 85); 14.3, 1.4 and 6.1 years after the acute encephalitis, respectively. Antibodies to other microbes were either undetectable in the CSF or the serum/CSF ratios were normal. The clinical sequelae in about half of the patients and the signs of intrathecal mumps antibody production are suggestive of a chronic process in the central nervous system after encephalitis associated with mumps virus infection.