The authors studied the relationship between total leukocyte count and body weight in 42 morbidly obese patients (weight range, 101.5-206.8 kg). None of the patients had a recent history of infection, hematologic disorders, chronic respiratory disease, or were smoking more than 5 cigarettes/day. The range of total leukocyte count was 4.2-12.0 X 10(9) cells/L. Twelve of the 42 patients had a leukocyte count of 10 X 10(9) cells/L. There was a significant correlation between body weight (kg) and total leukocyte count (r = 0.68, P less than 0.001). Morbid obesity should be considered as one of the causes of physiologic leukocytosis.