A population-based study of mumps orchitis over a 40-year period in Rochester, Minnesota, is reported. All medical records for patients with orchitis, atrophic testis, parotitis, and diagnosed or suspected mumps for the population of Rochester were reviewed, and detailed abstracts were prepared for all those with mumps orchitis. Information abstracted included the relationship of testicular to parotid involvement, the presence of unilateral or bilateral testicular involvement and subsequent atrophy, the presence of other complications of mumps, the treatment of mumps orchitis, and the presence of other major diagnoses. Also investigated was the number of subsequent congenital malformations in male offspring of these patients. The age distribution of the patients with orchitis (median age, 29 years) differed appreciably from those with mumps (median age, 8 years). There was no apparent increase in genitourinary malformations in the male offspring conceived after the occurrence of mumps orchitis in the fathers. Of the 132 men who had orchitis, 2 subsequently had testicular neoplasms. Both of these patients were in the group of 47 who were noted to have an atrophic testis after the occurrence of orchitis.