Nine of 24 ovariectomized nude mice developed ulcerative skin lesions 28 d after implantation with human breast cancer cells and slow-release estrogen pellets. Aerobic culture of samples from the skin lesions yielded Staphylococcus intermedius. By day 45 postimplantation, all mice displayed ulcerative skin lesions, and 5 mice exhibited hunched posture, listless behavior, cyanosis, anorexia, and dehydration. This subset was euthanized but not necropsied. When additional animals became moribund, the investigator elected to end the study at day 51. At necropsy, all 20 but 1 had cystitis and urolithiasis, characterized by relatively large struvite stones and crystals that had filled the urinary bladders of the research animals and caused severe thickening of the bladder walls. Gram-positive cocci were observed microscopically in both the kidneys and bladders of the necropsied animals. Spontaneous urolithiasis has not previously been documented to occur in association with infection of female nude mice by S. intermedius.