Familial isolated growth hormone deficiency (IGHD) has been associated with complete deletions of the hGH-N gene encoding the pituitary growth hormone (GH) in a large number of cases. However, there is still no alternative empirical explanation for the remaining familial or non-familial IGHD cases. We studied a large kindred including five IGHD-affected first cousins to determine possible IGHD inheritance and whether the hGH-N gene was the cause of IGHD in this pedigree. Sex-linked and autosomal recessive transmission of IGHD in this kindred was rejected. Autosomal dominant inheritance was the most probable explanation according to a model of one locus with two alleles, one being dominant for IGHD, under genetic modifiers or epistasis. Southern blotting analysis (BamHI and HindIII digestions) was used to determine whether the hGH-N gene was present in the patients and their family members. Because we found that the hGH-N gene was present, five restriction fragment length polymorphisms (RFLPs; HincII, MspI-A and B, and BglII-A and B) linked to the hGH-N gene were used to try to identify the possible RFLP haplotypes in the pedigree that could be markers or associated with the abnormal hGH-N alleles responsible for IGHD. From the haplotype analysis, it appeared that other genes not linked to the hGH-N gene cluster were the cause of the IGHD phenotype in this kindred. An alternative conclusion could be that the hGH-N gene was responsible for IGHD in this kindred, if a mutation (gene conversion) at the MspI-B site or a reciprocal recombination event between the HincII and MspI-B sites occurred from generation P to F1 and a similar event took place from generation F1 to F2. The non-significant GH responses of patients to the growth releasing factor test confirmed that the hGH-N gene structural product or some step in its regulation was responsible for causing IGHD in this kindred. We suggest that genetic micromutations in the hGH-N gene are present and are responsible for IGHD. We developed a method using the polymerase chain reaction to amplify a 790-bp fragment of the hGH-N gene. The fragment spanned from the second part of the dyad symmetry region in the non-transcribed 5' end of the hGH-N gene to 9 bp before the alternative splice-acceptor site in exon 3. The expected fragment was verified by its digestion with seven diagnostic restriction endonucleases (BamHI, FspI, PstI, NdeI, BssHII, BglII and HincII).(ABSTRACT TRUNCATED AT 400 WORDS)