Summary 1. The epidemiology, clinical course,and serologic findings in 30 cases of mumps meningoencephalitis occurring in children under 14 years of age are presented. 2. Thirteen patients did not haveparotitis at any time during the course of their meningoencephalitis. Of the remaining patients, 12 had parotitis prior to, 2 had parotitis concomitant with, and 3 had parotitis following the onset of symptoms of central nervous system involvement. 3. Fifty-five per cent of the cases of meningoencephalitis without parotitis occurred in the summer season. Forty-seven per cent of the cases of meningoencephalitis with parotitis occurred in the spring season. 4. A presumptive diagnosis ofmumps infection was made in 32 per cent of the cases of mumps meningoencephalitis without parotitis on the basis of the presence of a higher mumps antisoluble antibody titer than the mumps antivirus antibody titer in the initial serum drawn within the first 5 days of illness. 5. Confirmation of the etiological diagnosis was possible in all of the 13 cases of mumps meningoencephalitis without parotitis and in 10 of the 11 serologically studied cases with parotitis by demonstrating a significant rise in the complement fixation antibody titer against mumps soluble or mumps virus antigen in a second serum drawn at least 5 days days after the first serum was obtained. 6. No complications were observedduring the course of illness in any of the 30 cases. A convulsive disorder occurred in one patient 2 years after recovery from meningoencephalitis.