Previous studies have evaluated cellular proliferation indices, KIT expression, and c-kit mutations to predict the clinical behavior of canine mast cell tumors (MCTs). The study purpose was to retrospectively compare mitotic index, argyrophilic nucleolar organizer regions (AgNORs)/nucleus, Ki-67 index, KIT labeling pattern, and internal tandem duplication mutations in c-KIT between stage I and stage II grade II MCTs. Medical records and tumor biopsy samples from dogs with Grade II MCTs with cytological or histopathological regional lymph node evaluation were included. Signalment, tumor location and stage, and presence of a recurrent versus de novo tumor were recorded. Mitotic index, AgNORs/nucleus, Ki-67, KIT staining pattern, and internal tandem duplication mutations in exon 11 of c-KIT were evaluated. Sixty-six tumors (51 stage I; 15 stage II) were included. Only AgNORs/nucleus and recurrent tumors were significantly associated with stage (odds ratio 2.8, 95% confidence interval [CI] 1.0-8.0, P = .049; odds ratio 8.8, 95% CI 1.1-69.5; P = .039). Receiver-operator characteristic analysis showed that the sensitivity and specificity of AgNORs/cell ≥ 1.87 were 93.3% and 27.4%, respectively, (area under the curve: 0.65) for predicting stage. Recurrent tumors and higher AgNORs/nucleus are associated with stage II grade II MCTs; however, an AgNOR cutoff value that reliably predicts lymph node metastasis was not determined.