Eighty-four ovarian cancer (OCa) patients and 84 controls with nonmalignant conditions matched by age and ethnic origin were interviewed with regard to clinical mumps history and their sera were tested for complement fixation (CF) mumps antibodies. OCa patients differed from the controls in the response to past mumps infection in two respects: 1) They appeared to be more likely to have developed subclinical mumps as evidenced by a lower rate of clinical mumps history in the presence of serological evidence of similar infection rates among those with positive and those with negative clinical mumps history. 2) They tended to present lower persistent mumps CF antibody titers. These results may be interpreted to indicate that an immunological incompetence enables the development of OCa possibly through a direct etiologic role of mumps virus.