We examined directly the effects of a moderate dosage of caffeine (400 mg/d) on the calcium economy in 16 healthy premenopausal women in a double-blind, placebo-controlled crossover design. The subjects took divided doses of caffeine (100 mg/tablet) or identical-appearing placebos with decaffeinated coffee on personalized schedules for treatment periods lasting 19 d each and with 37-d interstudy intervals. We randomized the treatment sequence among subjects and studied them as inpatients under metabolic-balance conditions. We found no significant effects of caffeine on fractional calcium absorption, endogenous fecal calcium, or urine calcium, whether examined day by day or cumulatively. Although the mean balance shift was negative, the change was not significantly different from zero. There was evidence of altered bone remodeling, with slight decreases in bone accretion, bone resorption, and calcium pool turnover.