Pseudohypoparathyroidism type Ia (PHP type Ia) is associated with several endocrinopathies. We describe two siblings with PHP type Ia, hypothyroidism and growth hormone deficiency (GHD). To our knowledge, these patients are the first to have PHP type Ia and GHD. The defect which is most likely responsible for the three endocrinopathies is a deficiency of the stimulatory guanine nucleotide-binding regulatory protein, Gs.