Women with anorexia nervosa have reduced bone mass and may develop fractures. Neither the pathophysiology of this osteoporosis nor its natural history is known. To study the clinical course of osteoporosis, we followed up 27 women with anorexia nervosa for a median of 25 months (range, 9 to 53 months). At study entry, cortical bone density, measured by single-photon absorptiometry of the radial shaft, was low (mean ± SD, 0.63 ± 0.07 g/cm2) and inversely related to the duration of amenorrhea (r= -0.49). During follow-up, most patients gained weight (n = 19), took calcium supplements (n = 16), and exercised regularly (n = 22), but fewer than half reached 80% or more of ideal body weight (n = 11), resumed menses (n = 6), or received estrogen (n = 4). Cortical bone density was stable during follow-up for the group as a whole; the mean annual change ( ± SD) was + 0.005 ( ±.015) g/cm2 (95% confidence interval, - 0.0009 to +0.0109). There was no significant difference in the mean change in bone density between women who attained 80% of ideal weight and those who did not or between groups who did or did not regain menses, take estrogen or calcium, or exercise vigorously. Four fractures were clinically observed in three women during follow-up. The rate of 0.05 nonspine fractures per person-year (95% confidence interval, 0.02 to 0.13) exceeds that of normal women in this age range (relative risk, 7.1; 95% confidence interval, 3.2 to 18.5). We conclude that reductions in cortical bone density appear not to be rapidly reversed by recovery from anorexia nervosa and that anorectic women may have an increased risk of fracture.