We studied the relations between body height, body mass index (BMI), and fatal hip fractures prospectively in a large, representative population. During the years 1963-1975, a nationwide compulsory mass x-ray examination including standardized height and weight measurements took place in Norway covering all persons age 15 years and older. In the study presented here, we selected women (N = 357,807) and men (N = 316,041) age 50-89 years at screening. We matched the file to the national death register containing causes of death throughout 1991; we defined cases as persons with hip fracture mentioned on their death certificates. During an average follow-up of 16.4 years, we identified a total of 6,087 fatal hip fractures in the study population. There was a distinct inverse relation between BMI and fatal hip fracture, with an age-adjusted relative risk (RR) in the three highest vs the low quartile of 0.68 [95% confidence interval (CI) = 0.63-0.72] in women and 0.57 (95% CI = 0.52-0.62) in men. The risk of fatal hip fractures increased slightly with increasing body height [RR = 1.10 (95% CI = 1.04-1.16) in women and RR = 1.08 (95% CI = 1.01-1.16) in men per 10-cm increase in body height]. This study indicates that low BMI is an important risk factor for fatal hip fractures and that body height has a weak, positive association.