Complete blood counts are used frequently by physicians to assess and manage the development of complications of diseases. We studied 100 patients bitten by Bothrops jararaca snakes, and correlated their haematological values with the severity of envenoming and the development of complications. Patients who developed both local and systemic bleeding showed a greater drop in packed cell volume, red blood cell (RBC) count and haemoglobin concentration than those with who did not bleed. No morphological changes in RBCs were seen in blood films. Total white blood cell (WBC) counts were significantly higher in the clinically "more severe" group than in the "less severe" group on admission. Neutrophilic leucocytosis with left shift was present on admission, concurrently with a decrease in eosinophil and lymphocyte counts. These changes tend to become more marked 6h after antivenom therapy, and are greatest in "more severe" envenoming. Thrombocytopenia on admission is positively associated with the development of systemic bleeding and the severity of envenoming. Thrombocytopenia may also be a useful prognostic indicator for the development of local complications, such as necrosis. The intensity of neutrophilia and eosinopenia might be used to follow the progression of necrosis in victims of snake bite.