Familial male-limited precocious puberty is a male-limited autosomal dominant condition. It is characterized by increased testosterone synthesis in the absence of testicular stimulation by luteinizing hormone (LH). We hypothesised that an abnormal configuration of the LH receptor might autonomously activate G protein coupling, and thereby cause the overproduction of testosterone in this condition. To test this hypothesis, we screened for mutations in a part of the LH receptor gene that is important for G protein binding. DNA sequence variation was detected in 2 out of 5 families with male-limited precocious puberty by the single strand conformation polymorphism technique. Direct sequencing demonstrated different single nucleotide substitutions in the sixth transmembrane region of the LH receptor gene. The mutations cosegregated with the disorder in both families (lod score 5.76 without recombination). Both mutations cause an amino acid substitution in the sixth transmembrane domain, close to the C-terminal portion of the third cytoplasmatic loop, a region which is important for the binding of G proteins. We conclude that familial male-limited precocious puberty cosegregates with missense mutations in the LH receptor gene. These findings support the hypothesis that increased activity of the LH receptor is the pathogenetic mechanism that causes the abnormal pubertal development in this condition.