This article describes clinical follow-up of 15 patients—13 females and two males—with McCune-Albright syndrome. Osseous fractures occurred only during childhood, while hearing impairment due to temporal bone involvement occurred in four of six adults. Four females with precocious puberty had final heights that were not different from the mean for normal females; they eventually developed regular menses, and two had children. Persistent hyperthyroidism requiring ablative therapy occurred in three subjects, while hypophosphatemia occurred in three subjects. The protean manifestations of this disorder suggest that it results from a basic defect of cellular regulation. We postulate that its varied endocrine abnormalities result from altered regulation of intracellular cyclic adenosine monophosphate effects.