We conducted a meta‐analysis to summarize the available evidence from cohort studies on the association between excess body weight and incidence of leukemia. Studies were identified by searching the MEDLINE and EMBASE databases (1966–July 2007) and by examining the references of retrieved articles. A random‐effects model was used to combine the results from individual studies. We identified 9 cohort studies with data on body mass index (BMI) or obesity in relation to incidence of leukemia. Compared with nonoverweight individuals (BMI < 25 kg/m²), the summary relative risks (RRs) of leukemia were 1.14 [95% confidence interval (CI), 1.03–1.25] for overweight individuals (BMI 25–30 kg/m²) and 1.39 (95% CI, 1.25–1.54) for obese (BMI ≥ 30 kg/m²) individuals. On a continuous scale, a 5 kg/m² increase in BMI was associated with a 13% increased risk of leukemia (RR, 1.13; 95% CI, 1.07–1.19). In a meta‐analysis of 4 studies reporting results on subtypes of leukemia, the summary RRs associated with obesity were 1.25 (95% CI, 1.11–1.41) for chronic lymphocytic leukemia, 1.65 (95% CI, 1.16–2.35) for acute lymphocytic leukemia, 1.52 (95% CI, 1.19–1.95) for acute myeloid leukemia and 1.26 (95% CI, 1.09–1.46) for chronic myeloid leukemia. This meta‐analysis indicates that excess body weight is associated with an increased risk of developing leukemia. © 2007 Wiley‐Liss, Inc.