Overweight [body mass index (BMI) 25.0-29.9 kg/m²] and obesity (BMI ≥30 kg/m²) are risk factors for several malignancies. The Iowa Women's Health Study was examined to determine whether increased BMI was associated with leukemia development. Over 40,000 Iowa women (ages 55-69 years) completed a self-administered lifestyle and health questionnaire in 1986 that included current height and weight. Two hundred women developed leukemia during the period 1986 to 2001 including 74 acute myelogenous leukemia (AML) and 88 chronic lymphocytic leukemia. The risk of AML was increased among women who reported being overweight or obese (relative risk, 1.9; 95% confidence interval, 1.0-3.4; relative risk, 2.4; 95% confidence interval, 1.3-4.5; P_trend = 0.006) compared with women of normal weight. There was little evidence of a positive association for chronic lymphocytic leukemia (P_trend = 0.6). Given the prevalence of overweight and obesity in the United States, the population attributable risk of AML due to obesity could approach 30%.