Pseudohypoparathyroidism includes a genotypically diverse group of syndromes of primary resistance to hormones whose actions are mediated by cyclic adenosine 3':5'-monophosphate. In most cases, clinical presentation results from resistance to actions of parathyroid hormone, while resistance to other hormones remains subclinical. The largest subgroup of patients with this disorder, characterized by a typical somatic phenotype termed Albright's hereditary osteodystrophy, exhibits generalized deficient activity of a membrane protein that couples hormone receptors to stimulation of adenylate cyclase. Phenotypic expression of the disease varies greatly between families, within families, and even during the clinical course of individual patients. Genetic transmission occurs via several distinct loci, which produce different patterns of inheritance.