The distribution and correlates of bone mass among women ages 20-35 were studied. During young adulthood, bone mass reaches its maximum level and appears to stabilize. Factors that establish the individual's relative position in this plateau period may influence the ultimate expression of bone loss with aging. Mid-distal radial bone mass was measured in a geographically defined population of 86 women in two rural, demographically similar communities in Iowa. The water supplies provided 55 and 375 mg/liter of elemental calcium, generating significantly different mean community dietary calcium intakes of 871 and 1,233 mg/day, respectively. Bone mass was measured by single-beam photon densitometry and correlated with data from physical measurements, medical history, and reported nutritional intake. In evaluating the joint effect of variables in a multiple regression procedure, forearm bone mass was negatively and significantly associated with alcohol consumption (P = 0.0327) and with a first pregnancy prior to age 20 (P = 0.0250). There was a trend for current calcium intake estimated from 24-hr recall to be positively associated with bone mass (P = 0.0816). Because the 24-hr recall is characterized by significant error due to daily variability of an individual's intakes, a more general calcium intake was estimated from food frequency. Women whose estimated intake of calcium from food frequency was greater than 800 mg/day, the Recommended Daily Allowance, had significantly greater bone mass than women whose intake was estimated to be less than 800 mg/day (P = 0.0053). No relationship was observed with oral contraceptive use, parity, breastfeeding practices, smoking behavior, or measures of physical activity.