Forty-eight feline mammary carcinomas (FMC) were resected surgically from 48 cats to determine whether the Ki-67 index (Ki-67I) would provide an indication of the post-surgical survival time (PST). Twenty-four cats (50 per cent) were still alive (group A) one year after surgery, whilst 24 (50 per cent) (group B) had died. Formalin-fixed, paraffin wax-embedded histological sections were immunostained with a monoclonal antibody to Ki-67 (MIB-1) and at least 1000 nuclei in eight to 10 representative fields were counted. The Ki-67I was expressed as the percentage of positive nuclei. In FMC, the Ki-67I ranged from 7.5 to 49.7 (24.8+/-9.5). A statistically significant difference (P = 0.000006) in the Ki-67I was found between group A and group B cats. No other statistically significant differences were found between these groups. The Ki-67I did not correlate with age or different histological type, according to the WHO classification. A Ki-67I cut-off of 25.2 represents a useful tool for identifying FMC with a more aggressive course.