Platelet counts were performed in 43 Cavalier King Charles Spaniels (CKCS, a breed predisposed to macrothrombocytopenia) and in 10 control dogs using 3 automated systems and 3 manual methods (erythrocytelysing agents + counting chamber or evaluation of blood smear). Good correlations were found between platelet counts using all methods (all P < 0.0001; R² = 0.71-0.85). Best correlations were found between the manual methods. Significantly larger platelets were found in CKCS with platelet count <100,000/μl when compared with control dogs and CKCS with platelet count >100,000/μl (both P < 0.0001). All platelet counts - except when made with the 2 counting chamber methods - were underestimated at platelet counts ≤100,000/μl.