The morphology and biology of 179 malignant mammary tumors from 170 cats were described. The site of tumors was nearly equally distributed among the 4 pairs of glands. The recorded sex distribution was 114 intact Females, 40 ovariectomized females (average age at spaying, 5.8 years), and 2 castrated males. The average age at first detection was 10.8 years, and average survival time aFter detection was 12.3 months; this poor prognosis was probably related to an average delay of 7 months beFore the first operation and the correlation of the 1-year survival with the tumor volume at first detection. OF the 129 necropsies,120 showed metastases most commonly in drainage lymph nodes (82.8%), lungs (83.6%), pleurae (42.2%), and livers (23.6%). Subdivision by histologic pattern showed 53 tubular adenocarcinomas, 52 papillary adenocarcinomas, 35 solid carcinomas, 2 mucoid carcinomas, 34 compound carcinomas, and 3 sarcomas. Histologic grading revealed that 50% of the low-grade malignancy group survived more than 1 year in contrast to 10% of the high grade group.