An 8-year-old, male castrated Labrador Retriever presented with coughing, inappetence, lethargy and fever. Two days prior to presentation, one dose of fenbendazole had been given for chronic, intermittent diarrhea. Routine hemogram showed a severe neutropenia and moderate thrombocytopenia, while bone marrow aspirate cytology revealed bone marrow hypoplasia. Supportive care, including fluid therapy initially and broad-spectrum antibiotics for several weeks, resulted in complete resolution of clinical signs and hematologic abnormalities. An adverse drug reaction to fenbendazole was suspected based on the absence of detectable infectious and neoplastic causes as well as resolution of hematologic abnormalities following drug withdrawal.