The molecular basis and the locus responsible for most familial cases of isolated GH deficiency (IGHD) are still unknown. The GH-releasing hormone (GHRH) gene has been evaluated as a possible candidate in 23 unrelated families with IGHD, 14 of whom were classified as having autosomal recessive IGHD type IB and 9 of whom had autosomal dominant IGHD type II. Three highly polymorphic microsatellites (dinucleotide repeats), mapped close to GHRH on chromosome 20 by previous linkage studies, were analyzed as markers for the GHRH locus. All available family members were genotyped for D20S44 [no recombination with GHRH at a LOD (logarithm of the odds) score of 3.6]. Noninformative families were also genotyped for D20S45 and/or D20S54 (located at approximately 1 and 3 centiMorgan of genetic distance from GHRH, respectively). Twenty families were informative for linkage analysis with 1 or more of these markers. We found at least 1 obligate recombinant with discordance between phenotype and genotype in 19 of the 23 families (83%). There is only a very small chance (1-3% or less) that the discordances observed are due to recombination between the GHRH locus and the marker tested. Concordant segregation was seen in only 1 type IB family (4%). When probands from this and the 3 noninformative families were screened for sequence variants in the 5 exons of the GHRH gene by single strand conformation analysis, no abnormal patterns were observed. We conclude that mutations responsible for IGHD are not within or near the structural gene for GHRH on chromosome 20 in the 23 families studied. As linkage to the GH-1 gene has also been previously excluded in 65% of these families, mutations in a locus or loci unlinked to GH-1 and GHRH must be responsible for the majority of these IGHD families.