This cross-sectional study of the Rancho Bernardo, California, cohort examines the relation between bone mineral density and eight measures of body size (total weight, body mass index, waist-hip ratio, lean mass, fat mass, percentage fat mass, and current and maximum adult height) measured between 1988 and 1991 in 1,492 ambulatory white adults aged 55-84 years. Lean mass, fat mass, and percentage fat mass were measured by bioelectric impedance. Bone mineral density was measured at the hip and lumbar spine with dual-energy x-ray absorptiometry and at the midshaft and ultradistal radius with single photon absorptiometry. In multiple linear regression models adjusted for age, smoking, exercise, alcohol, thiazide use, and estrogen use (in women), total weight was the most consistent marker of bone mineral density overall. In this cohort, all measures of body size were associated with bone mineral density in both sexes and were better markers of bone mineral density in the weight-bearing sites than in the non-weight-bearing sites, implying a mechanical effect of weight on bone mineral density.