Viral etiology of clinically typical parotitis was studied by virus isolation and serology in 100 consecutive patients presented at the Department of Infectious Diseases, University of Turku, during a period of over 2 years. Mumps etiology was confirmed in 95 cases and in one case a probable parainfluenza type 3 infection was diagnosed by IgM serology. In the remaining 4 patients no etiological evidence of any of the viruses studied was obtained. When different methods to diagnose an acute mumps infection were compared, the most sensitive was the assay for specific IgM antibodies.