There is still a debate about the role of body mass index (BMI) as a risk factor for all-cause mortality. Most investigations with large sample sizes focused on populations from the United States, studies from Central-European cohorts are not available. We investigated the association between BMI and all-cause mortality and cause-specific mortality within a cohort in Austria. Design of this article is "Cohort study". The Subjects used were 184,697 men and women (mean age 41.7 +/- 15.4 years). Weight and height were measured. Cox proportional hazards models were used to estimate hazard ratios (HR). During a median follow-up of 15.1 years 15,557 deaths (6,077 from cardiovascular disease, 4,443 from cancer and 606 from respiratory disease) were seen. A U-shaped association between BMI and all-cause mortality was observed in men and women. Compared with the reference category (BMI 22.5-24.9 kg/m(2)) high risks were found both in the highest category of BMI (> or =35 kg/m(2)) with HR of 2.13 (95% CI, 1.82-2.48) in men and 1.60 (95% CI, 1.42-1.81) in women and in the lowest category (<18.5 kg/m(2)) with HR of 2.57 (95% CI, 2.17-3.05) in men and 1.40 (95% CI, 1.21-1.62) in women. Similar patterns were seen among ever-smokers and non-smokers. Increased mortality with increasing BMI was driven by cardiovascular diseases and to a lesser extent by cancers. Respiratory diseases contributed to mortality in the lowest BMI category independently from smoking status. Underweight and obesity were both associated with higher all-cause mortality in men and women.