Circulating mast cells were not identified in clinically normal cats or ill cats without mast cell tumor-related disease. Ill cats did have conditions that caused eosinophilia, basophilia, or neutrophilia. The absence of mast cells in buffy coats obtained from clinically normal and ill cats lends support to the current practice of buffy coat evaluation for tumor staging and follow-up examinations in cats with mast cell tumors. Further studies of buffy coat analysis in cats with different forms of mast cell tumors are indicated to specifically elucidate the test's prognostic value for those patients.