Most patients with autosomal dominant pseudohypoparathyroidism type Ib (AD‐PHP‐Ib) carry maternally inherited microdeletions upstream of GNAS that are associated with loss of methylation restricted to GNAS exon A/B. Only few AD‐PHP‐Ib patients carry microdeletions within GNAS that are associated with loss of all maternal methylation imprints. These epigenetic changes are often indistinguishable from those observed in patients affected by an apparently sporadic PHP‐Ib form that has not yet been defined genetically. We have now investigated six female patients affected by PHP‐Ib (four unrelated and two sisters) with complete or almost complete loss of GNAS methylation, whose healthy children (11 in total) showed no epigenetic changes at this locus. Analysis of several microsatellite markers throughout the 20q13 region made it unlikely that PHP‐Ib is caused in these patients by large deletions involving GNAS or by paternal uniparental isodisomy or heterodisomy of chromosome 20 (patUPD20). Microsatellite and single‐nucleotide variation (SNV) data revealed that the two affected sisters share their maternally inherited GNAS alleles with unaffected relatives that lack evidence for abnormal GNAS methylation, thus excluding linkage to this locus. Consistent with these findings, healthy children of two unrelated sporadic PHP‐Ib patients had inherited different maternal GNAS alleles, also arguing against linkage to this locus. Based on our data, it appears plausible that some forms of PHP‐Ib are caused by homozygous or compound heterozygous mutation(s) in an unknown gene involved in establishing or maintaining GNAS methylation. © 2011 American Society for Bone and Mineral Research