Overweight and obesity increase the risk of numerous chronic diseases, including several forms of cancer. However, the association between excess body weight and all-cause mortality among young and middle-aged women is incompletely known, and the impact of menopausal status on the association has hardly been investigated. We studied prospectively a cohort comprising a population sample of 102,446 women from Norway and Sweden aged 30-50 years when they answered an extensive questionnaire in 1991/1992. During follow-up through year 2000, 1187 women in the cohort died. We used Cox proportional hazard models to estimate multivariate Hazard rate ratios (HRR) with 95% confidence intervals (CI) of death in relation to body mass index (BMI, weight (kg)/height (m(2))) at start of follow-up. Both in age-adjusted models and in models adjusting for several variables (including smoking and physical activity) mortality increased with increasing BMI among premenopausal women, whereas a U-shaped relationship was seen among the postmenopausal women. Among premenopausal women obesity (BMI 30.0) doubled the mortality (HRR = 2.2, 95% CI: 1.7-3.0) when compared to women of normal weight (BMI 18.5-24.9), whilst the association was modest after menopause. Although we had limited power to analyze women who were underweight (BMI<18.5), an excess mortality of about 50% was seen among postmenopausal women. No excess risk was found for underweight premenopausal women. The data indicate that the rapidly growing prevalence of obesity in many Western countries will substantially increase premature deaths among young women.