Background: Thrombocytopenia is the most common and consistent hematologic finding in patients with canine monocytic ehrlichiosis. Dogs that recover from the severe thrombocytopenia still show bleeding tendencies, which suggest that platelet dysfunction is present. Objectives: The purpose of this study was to determine the occurrence and duration of platelet dysfunction in dogs with ehrlichiosis and to assess whether dysfunction is related to thrombocytopenia. Methods: Ten adult male and female mongrel dogs were used in the study; 7 were inoculated intravenously with whole blood containing Ehrlichia canis, and 3 were used as controls. Platelet aggregation (with collagen/epinephrine and adenosine diphosphate (ADP)/epinephrine) and platelet counts were evaluated weekly for 112 days. Results: The infected group showed a decrease in platelet aggregation response to collagen/epinephrine and ADP/epinephrine on days 7, 14, 21, 28, and 35 (P < .05). Thrombocytopenia was observed in all infected animals from day 7 to 35 postinfection (P < .05). Conclusions: The tendency of dogs infected with E canis to bleed may be related not only to thrombocytopenia but also to platelet dysfunction associated with the disease.