Obesity and underweight in young adulthood are associated with greater risk of future disability pension. Neither underlying causes of disability pension nor whether overweight confers excess risk is established in this age group. The aim of this study was to investigate risk of future disability pension according to body mass index (BMI) in young adulthood. BMI was measured at military conscription (1969-1994; n=1 191 027; mean age 18.3+/-0.5 years). Date and cause of disability pension, death and emigration dates were collected from national registers (1971-2006). Muscular strength, age, municipality, socioeconomic position, testing center and year were adjusted for in Cox regressions. During 28.4 million person-years, 60 024 subjects were granted disability pension. The hazard ratios (HRs) for underweight (1.14, CI 95% 1.11-1.17), overweight (1.36, 1.32-1.40), moderate (BMI 30-34.9; 1.87, 1.76 to 1.99) and morbid obesity (BMI>or=35; 3.04, 2.72-3.40) were elevated compared to normal weight. Not adjusting for muscular strength led to overestimation of the risk in underweight (1.27, 1.24-1.31), but underestimation in overweight (1.29, 1.25-1.33), moderately (1.72, 1.62-1.82) and morbidly obese subjects (2.77, 2.48-3.09). For circulatory and musculoskeletal causes, respectively, HRs were elevated only for overweight (2.06; 1.82-2.34; 1.47; 1.39-1.55) and obesity (3.51; 2.79-4.40; 2.15; 1.94-2.38). The same applied for tumors and nervous system, but not psychiatric causes, for which underweight (1.20; 1.16-1.24) displayed similar HR as overweight (1.21; 1.16-1.27), whereas the risk in obese subjects was higher (1.60; 1.46-1.75). The risks in overweight and obese, but not underweight, subjects were significantly elevated for each cause investigated. Although causality cannot be inferred, productivity losses associated with adverse BMI in young adulthood appear to be large.