Objective: Pseudohypoparathyroidism type Ia (PHP-Ia) is a hereditary disorder characterized by resistance to multiple hormones that work via cAMP such as PTH and TSH, accompanied by typical skeletal features including short stature and brachydactyly, termed Albright hereditary osteodystrophy (AHO). In affected kindreds, some members may have AHO but not hormone resistance; they are termed as pseudopseudohypoparathyroidism (PPHP). The molecular basis for the disorder is heterozygous inactivating mutation of the Gsα gene. In affected families, subjects with both PHP-Ia and PPHP have the same Gsα mutations. The skeletal features common to PPHP and PHP-Ia are presumably caused by tissue-specific Gsα haploinsufficiency. Other features that distinguish between PPHP and PHP-Ia, such as the multihormone resistance, are presumably caused by tissue-specific paternal imprinting of Gsα. This suggests that major differences in phenotype between PHP-Ia and PPHP point to specific tissues with Gsα imprinting. One such major difference may be cognitive function in PHP-Ia and PPHP. Design: Description of a large family with PHP-Ia and PPHP. Patients: Eleven affected subjects with PHP-Ia or PPHP in one family. Measurements: Cognitive impairment (CI) was defined by a history of developmental delay, learning disability and the Wechsler intelligence scale. Results: CI occurred only in the five PHP-Ia but not in the six PPHP subjects. Hypothyroidism which occurred in all PHP-Ia subjects was apparently not the cause of CI as it was mild, and was treated promptly. Analysis of additional Israeli cases, and the published cases from the literature, all with documented Gsα mutations, revealed that CI is prevalent in PHP-Ia [60 of 77 subjects (79%)] but not in PPHP [3 of 30 subjects (10%)] (P < 1 × 10 -6). Conclusion: We suggest that Gsα is imprinted in the brain.