We examined the relationship of body mass index (BMI), diabetes and smoking to endometrial cancer risk in a cohort of 36 761 Norwegian women during 15.7 years of follow-up. In multivariable analyses of 222 incident cases of endometrial cancer, identified by linkage to the Norwegian Cancer Registry, there was a strong increase in risk with increasing BMI (P-trend <0.001). Compared to the reference (BMI 20-24 kg m(-2)), the adjusted relative risk (RR) was 0.53 (95% confidence interval (CI): 0.19-1.47) for BMI<20 kg m(-2), 4.28 (95% CI: 2.58-7.09) for BMI of 35-39 kg m(-2) and 6.36 (95% CI: 3.08-13.16) for BMI>or=40 kg m(-2). Women with known diabetes at baseline were at three-fold higher risk (RR 3.13, 95% CI: 1.92-5.11) than those without diabetes; women who reported current smoking at baseline were at reduced risk compared to never smokers (RR 0.55, 95% CI: 0.35-0.86). The strong linear positive association of BMI with endometrial cancer risk and a strongly increased risk among women with diabetes suggest that any increase in body mass in the female population will increase endometrial cancer incidence.