A population-based case-control study of lymphomas in England collected height and weight details from 699 non-Hodgkins lymphoma (NHL) cases and 914 controls. Obesity, defined as a body mass index (BMI) over 30 kg m(-2) at five years before diagnosis, was associated with an increased risk of NHL (OR = 1.5, 95% CI 1.1-2.1). The excess was most pronounced for diffuse large B-cell lymphoma (OR = 1.9, 95% CI 1.3-2.8). Genetic variants in the leptin (LEP 19G > A, LEP -2548G > A) and leptin receptor genes (LEPR 223Q > R), previously shown to modulate NHL risk, as well as a polymorphism in the energy regulatory gene adiponectin (APM1 276G>T), were investigated. Findings varied with leptin genotype, the risks being decreased with LEP 19AA (OR = 0.7, 95% CI 0.5-1.0) and increased with LEP -2548GA (OR = 1.3, 95% CI 1.0-1.7) and -2548AA (OR = 1.4, 95% CI 1.0-1.9), particularly for follicular lymphoma. These genetic findings, which were independent of BMI, were stronger for men than women.