MARIA DIMOPOULOU, DVM 1,2 , JOLLE KIRPENSTEIJN, Prof, DVM, PhD, Diplomate ACVS & ECVS 1,2 , HESTER MOENS, DVM 1,2 , and MARJA KIK, DVM, PhD 1,2 1 Department of Small Animal Clinical Sciences, Faculty of Life Sciences, University of Copenhagen, Copenhagen, Denmark; and 2 Departments of Clinical Sciences of Companion Animals, and Pathobiology, Faculty of Veterinary Medicine, Utrecht University, U Vet Surg 2008 37 6 466-71 ABSTRACT Objective—To investigate the histologic characteristics of feline osteosarcoma (OS) and compare the histologic data with phenotypically comparable canine OS. The effects of histologic and clinical variables on survival statistics were evaluated. Study Design—Retrospective study. Animals—Cats (n=62) and dogs (22). Methods—Medical records of 62 cats with OS were reviewed for clinically relevant data. Clinical outcome was obtained by telephone interview. Histologic characteristics of OS were classified using a standardized grading system. Histologic characteristics in 22 feline skeletal OS were compared with 22 canine skeletal OS of identical location and subtype. Prognostic variables for clinical outcome were determined using multivariate analysis. Results—Feline OS was characterized by moderate to abundant cellular pleomorphism, low mitotic index, small to moderate amounts of matrix, high cellularity, and a moderate amount of necrosis. There was no significant difference between histologic variables in feline and canine OS. Histologic grade, surgery, and mitotic index significantly influenced clinical outcome as determined by multivariate analysis. Tumor invasion into vessels was not identified as a significant prognosticator. Conclusion—Feline and canine skeletal OS have similar histologic but different prognostic characteristics. Clinical Relevance—Prognosis for cats with OS is related to histologic grade and mitotic index of the tumor.