Background and objecttives: The aims of this study were to determine whether: i) diseases in hospitalised South African dogs with leukocyte counts ≥35x109/l were different from, ii) hospitalisation time longer than and mortality rate higher than control dogs; iii) glucocorticoid treatment contributed to significant leukocytosis; iv) hypoalbuminaemia and thrombocytopaenia added prognostic value, v) high leukocyte counts predict complicated babesiosis. Methods: Records were examined from 182 hospitalized dogs with a WBC ≥35Χ109/l (LCG) and 179 hospitalized dogs with 3.0 ≤ WBC ≤30Χ109/l and immature neutrophil count ≤0.5Χ109/l (CG). Diagnoses were assigned to groups Infection, Immune-mediated; Necrosis; Neoplasia; Babesiosis; Other. Results: WBC, neutrophil count, lymphocyte count and monocyte count were higher in LCG than CG (p<0.0001) while eosinophil count was lower in LCG than CG (p<0.0001). Hct, platelet count, and serum albumin concentration were lower in LCG than CG (p<0.0001). There was no difference in neutrophil count, lymphocyte or monocyte count between glucocorticoid-treated and non-glucocorticoid-treated dogs in LCG. Disease frequencies differed significantly (LCG > CG) in Infection, Necrosis, Babesiosis and immune mediated haematological disease groups. The frequency of complicated babesiosis cases was higher in LCG in than in CG (p < 0.0001). Time of hospitalization was significantly (p<0.0001) longer for LCG than for CG. There was a significant relationship between total and immature neutrophil count and survival (p=0.01) Conclusions: Leukocytosis is more likely to indicate infection, complicated babesiosis, immune mediated hematological disease or necrosis in the population of dogs examined. Hypoalbuminaemia and thrombocytopaenia in an animal with significant leukocytosis is not prognostically useful, while the combination of total and immature neutrophil count is. In hospitalized animals with severe leukocytosis, glucocorticoid treatment does not increase the leukocyte count. / Dissertation (MMedVet)--University of Pretoria, 2007. / Companion Animal Clinical Studies / unrestricted