Ovarian cysts and one ovarian tumour, uterine tumours, vaginal calculi, abscesses of the broad ligament or undetermined testicular lesions were observed in 25 out of 502 female and male dusky dolphins (Lagenorhynchus obscurus) caught off Peru in 1985-1987 or 1993-1994. Tentative or definitive diagnoses included Graafian follicle cysts, luteinized cyst, ovarian parasitic granulomatous inflammation, dysgerminoma, leiomyoma, fibroleiomyoma and chronic fibrino-suppurative inflammation of the broad ligament. All lesions described represented first reports for L. obscurus, and the diagnosis of dysgerminoma was the first in a cetacean. It is also the first time that trematode eggs have been reported in the ovaries of cetaceans and that a vaginal calculus has been encountered in a sexually immature cetacean. The finding of struvite as a major component in two vaginal calculi suggested an infectious aetiology. Of 11 mature females with ovarian tumour or cysts or uterine tumours, only one (9.1%) was pregnant, i.e., significantly less than the expected pregnancy rate (53.5% in a random sample of Peruvian dusky dolphins). Several females with ovarian or uterine lesions and males with aberrant testes were large animals. It is possible that some of these lesions were associated with normal senescence of the reproductive system. (C) 2000 Harcourt Publishers Ltd.