High body mass index (BMI) has consistently been associated with increased colon cancer risk in men, but not in women. It is hypothesised that menopause-related changes in oestrogen levels play a role in gender-specific risk patterns. Most studies have been conducted in Western countries, where high incidence rates are coupled with a high prevalence of obesity and relatively common use of hormone replacement therapy (HRT) in post-menopausal women. This study evaluated the correlation between body mass index (BMI) and colon cancer risk in a relatively lean population, comprising 931 cases and 1552 controls, in Shanghai, China, where HRT use was extremely rare among women, during 1990–1993. Among men, colon cancer risk significantly increased with increasing BMI (P-trend = 0.005). Among women, the risk varied with age and menopause status in a similar pattern. Within each menopause stratum, however, the BMI-related risk was similar for those aged under 55 years and those aged 55 years and over, indicating a menopause rather than age effect. Among pre-menopausal women, the odds ratios (ORs) for subjects in the highest versus lowest quintile were 1.9 (95% CI 1.1–4.9) for those under 55 years of age, and 2.2 (95% CI 1.4–8.2) for those aged 55 years and over. Among post-menopausal women, the corresponding ORs were 0.6 (95% CI 0.5–0.91) and 0.7 (95% CI 0.5–0.95), respectively. Our findings suggest that BMI predicts colon cancer risk in both genders. Among women, however, the risk is modified by menopause status, possibly through altered endogenous oestrogen levels.