BACKGROUND Hematological and biochemical values in cats naturally infected by feline immunodeficiency virus (FIV) or feline leukemia virus (FeLV) are not completely documented. OBJECTIVE Report differences in laboratory values between FIV- or FeLV-infected and noninfected and between FIV- and FeLV-infected cats. METHODS Three thousand seven hundred and eighty client-owned cats tested for FIV and FeLV. METHODS Retrospective study. Evaluation of clinicopathologic changes in cats with defined FIV and FeLV status and for which laboratory data were available. RESULTS FIV-infected cats were more likely to be neutropenic (odds ratio [OR]=3.6, 95% confidence interval [95% CI] 2.1-6.2, P < .0001) and had lower serum activities of aspartate aminotransferase and glutamate dehydrogenase than control cats; serum total protein (8.1 +/- 1.1 versus 7.6 +/- 1.3 g/dL, P < .001) and gamma-globulin concentrations (2.2 +/- 1.1 versus 1.7 +/- 1.3 g/dL, P < .001) were higher than in uninfected cats. Compared with controls, FeLV-infected cats had a higher risk of anemia (OR = 3.8, 95% CI 2.4-6.0, P < .0001), thrombocytopenia (OR = 5.0, 95% CI 3.0-8.4, P < .0001), neutropenia (OR = 3.6, 95% CI 2.1-6.1, P < .0001), lymphocytosis (OR = 2.8, 95% CI 1.6-4.8, P= .0002), and lower erythrocyte counts (6.13 +/- 2.95 x 10(3) versus 8.72 +/- 2.18 x 10(3)/microL, P < .001), thrombocyte counts (253.591 +/- 171.841 x 10(3) versus 333.506 +/- 156.033 x 10(3)/microL, P < .001), hematocrit (28.72 +/- 12.86 versus 37.67 +/- 8.90%, P < .001), hemoglobin and creatinine concentration. CONCLUSIONS Hematologic abnormalities are common in FeLV-infected but not in FIV-infected cats. Clinicopathologic abnormalities are less frequent in FIV-infected cats and might reflect an unspecific immunologic response.