To the Editors: Dogs have a unique risk of developing cutaneous mast cell tumors (MCTs), which account for 7-21% of all canine skin tumors.1-4 Furthermore, MCTs are reported to be the most common canine skin tumors submitted for histology and diagnosed in veterinary teaching hospital populations,1,2,5,6 with an incidence of 129 in 100,000 dogs per year in one U.K. study of insured dogs.7 Several factors have been used to predict the biologic behavior of MCTs, including clinical stage, growth rate, systemic signs, anatomic location, histologic grade, number of argyrophylic nucleolar organizer regions, presence of proliferating cell nuclear antigen and number of Ki-67 positive nuclei.8,9 Among these, histologic grade is the most reliable in predicting survival.8,10-12 However, reproducibility problems lead to as much as 50-60% discordance between experienced pathologists.13,14 We have analyzed cytologic samples from 24 canine spontaneous cutaneous MCTs. Postoperative follow-up information was collected for a period of 40 weeks in all cases. Lymph node, spleen, liver and bone marrow status with respect to the presence of mast cells detected by use of cytology also was recorded. Presence of mast cells in any of those locations was considered metastasis. Neoplastic cells were preoperatively obtained by fine needle aspiration biopsy, fixed immediately with Merckofix spray (Merck, Darmstadt, Germany) and stained with Hemacolor (Merck). After being surgically removed, several tissue blocks from each tumor were formalin fixed and paraffin wax embedded, and sections (4 pm) were stained with hematoxylin-eosin (H-E) and toluidine blue, respectively.15 Subsequently all the tumors' diagnoses were confirmed with histopathology according to the World Health Organization International Histological Classification of Tumours of Domestic Animals.16 They were classified as grades 1-3 on the basis of the histopathologic grading criteria of Patnaik et al.11 Material for cytopathologic processing was analyzed using a Motic Professional B3 digital microscope (Wetzlar, Germany) coupled to a computer equipped with the Image Pro Plus analysis system (Media Cybernetics, Silver Spring, Maryland, U.S.A., version 4.5.0.29, for Windows 98/NT/2000 [Microsoft, Redmond, Washington, U.S.A.]). Briefly, for each case, 10 microscopic fields containing neoplastic cells were randomly selected. For each tumor 100 nuclei of cells from selected areas were measured. Overlapping and fragmented nuclei, nuclear caps and nuclei showing unclear boundaries were rejected.