OBJECTIVE: To examine the association between fitness, BMI, and neutrophil, lymphocyte, monocyte, basophil, and eosinophil concentrations in apparently healthy, nonsmoking men. DESIGN: Cross-sectional study of 452 men from the Aerobics Center Longitudinal Study examining the resting concentration of white blood cell subfractions across fitness (maximal METS during a treadmill exercise test) and fatness (BMI) categories after adjusting for age. RESULTS: Fitness was inversely associated with all WBC subfraction concentrations. After further adjusting for BMI, only total WBC, neutrophil, and basophil concentrations remained significantly associated with fitness. BMI was directly associated with total WBC, neutrophil, lymphocyte, monocyte, and basophil concentrations and when fitness was added to the model, only monocytes lost significance. CONCLUSION: Fitness (inversely) and fatness (directly) are associated with WBC subfraction populations.