Vinblastine toxicity is poorly documented in dogs. The aim of this study was to investigate the haematological alterations in dogs treated with vinblastine and prednisolone. Fourteen dogs with mast cell tumours (MCT) were selected on at least one of the following criteria: lymph node infiltration, surgical margin infiltration, grade II MCTs with Ki-67 >10%, and grade III MCTs. Starting 15 days after surgery, the dogs were given vinblastine (2 mg/m2 i.v. four times weekly, then twice monthly for 2 months) and prednisolone (2 mg/kg/day p.o.). An EDTA blood sample was collected weekly for complete blood count (CBC). A total of 98 doses of vinblastine were given to the 14 dogs and 114 CBC were performed. Abnormal haematological findings were observed in 12 CBCs from five dogs, which represent a prevalence of 20% of the total CBCs performed in these animals. The most prevalent abnormal finding was thrombopenia (9/12) most often with grade I toxicity (6/9). In conclusion, the risk of occurrence of adverse haematological effects resulting from vinblastine-prednisolone treatment seems limited in dogs with MCT and it should not be overestimated.