Mammary tumors are the third most common type of neoplasia in female cats. They are usually malignant and progress rapidly. Grading mammary tumors is useful in predicting postsurgical outcome. A recent study in human medicine shows a positive correlation between cell proliferation or tumor grade and apoptosis. This study was conducted to evaluate the role of apoptosis in feline mammary tumors and its relationship with invasiveness and cell proliferation assessed by mitotic index. Thirty-tree tissue samples from cats with malignant primary mammary tumors were evaluated. The neoplasias included several types of carcinoma. Thoracic radiographs and ultrasonograms of the spleen, liver, and kidneys were taken before surgery and every 2 months over a 2-year period. Follow-up was for 2 years or until death due to the tumor.