Reports of associations between body size and composition and risk of lymphohematopoietic malignancies have been inconsistent. In a prospective study of 40,909 people aged 27-75 years and followed up for an average of 8.4 years, we measured fat mass and fat-free mass (using bioelectrical impedance analysis) and measured waist circumference directly. All malignancies were ascertained via the population cancer registry. The risk of myeloid leukemia was positively associated with body mass index (compared with those < 25 kg/m2, overweight and obese persons' hazard ratios [HRs] = 5.3, 95% confidence interval [CI] = 1.9 to 15.2 and HR = 5.0, 95% CI = 1.6 to 15.5, respectively; P = .006), fat-free mass (per 10-kg increase HR = 1.83, 95% CI = 1.15 to 2.90; P = .01), and waist circumference (per 10-cm increase HR = 1.35, 95% CI = 1.06 to 1.72; P = .02). Lymphoproliferative malignancies and subgroups showed little relationship with body size.