Hyperprolactinemia occurs in 25 to 30 percent of young women with amenorrhea, and this condition is known to be associated with osteopenia. To determine whether the osteopenia is affected by treatment of hyperprolactinemia, we studied 32 women with hyperprolactinemic amenorrhea prospectively for 12 to 72 months to investigate the effects of sustained hyperprolactinemia or return of gonadal function on bone mass. We studied 18 patients using direct photon absorptiometry before and after normalization of serum prolactin levels. Initial bone densities ranged from 0.55 to 0.77 g per square centimeter (mean +/- 1 SD, 0.64 +/- 0.05)--densities significantly lower (P less than 0.001) than those of controls (0.71 +/- 0.04 g per square centimeter). After therapy, bone density increased significantly (P less than 0.001), to 0.67 +/- 0.05 g per square centimeter, but remained lower (P less than 0.05) than normal. Fourteen patients were followed without therapy. Their initial bone densities ranged from 0.62 to 0.75 g per square centimeter (mean, 0.67 +/- 0.04)--values significantly lower (P less than 0.02) than those in controls. There was a significant decrease (P less than 0.002) in bone density over time in this group. We conclude that (1) treatment of hyperprolactinemia increases bone mass in most amenorrheic women with osteopenia, (2) normalization of serum prolactin levels in such women is associated with prevention of bone loss, and (3) a subset of untreated women with hyperprolactinemia have progressive osteopenia, which could have adverse long-term health consequences.