To assess the effect of dietary calcium intake on risk of hip fracture, a geographically defined caucasian population in southern California was studied prospectively. Between 1973 and 1975, a quantified 24 hour diet recall was obtained by a dietician from 957 men and women aged 50 to 79 years at baseline. Follow-up to 1987 with mortality records and interviews showed 15 men and 18 women with hip fractures. The age-adjusted risk of hip fracture was inversely associated with dietary calcium whether considered as mg per day or as nutrient density (mg per 1000 kcal). No other nutrient was consistently associated with hip fracture in any Cox proportional hazards model that included calcium. The association between calcium and fracture persisted after adjustment for cigarette smoking, alcohol intake, exercise, and obesity. The significant independent inverse association of dietary calcium with subsequent risk of hip fracture (relative risk = 0.6 per 198 mg/1000 kcal) strongly supports the hypothesis that increased dietary calcium intake protects against hip fracture.