Uropathy associated with hydronephrosis was observed frequently in our male KK-Ay mouse colony during a long-term study of diabetes. The lesion occurred in 24 of the 31 KK-Ay male mice and accounted for the greatest number of spontaneous deaths among them. It was observed after 4 months of age and involved about hard plugs of altered seminal material resembling the seminal vesicle secretion. The plugs became impacted in the urethral bulb and the bladder. The penile anatomy, with its flexure, pressure on the urethra from the bulbocavernosus muscle, and the characteristic ability of the seminal fluid to easily coagulate to form the vaginal plug may have contributed to the lesion. Correlation between development of the uropathy and diabetes has not been established.