Recently, we reported two homozygous inactivating nonsense and missense mutations in exon 11 of the LH receptor (R) gene in 2 families: the first family had three 46,XY patients with female external genitalia due Leydig cell hypoplasia (LCH), and in their sister 46,XX patient with amenorrhea; the second family had one boy with micropenis due LCH. We now have studied the LHR gene in three additional unrelated Brazilian families with LCH. The first family, a consanguineous marriage, included a 46,XY phenotypically female patient with LCH and a 46,XX patient with normal breast development, amenorrhea and infertility. The clinical and laboratory findings of this patient were similar to the female patient with the previously described nonsense mutation. Basal and GnRH-stimulated levels of LH (28 and 105 IU/L) and FSH (21 and 32 IU/L) were elevated, and basal estradiol level was 42 pg/ml. Pelvic ultrasound revealed a hypoplastic uterus (26 ml) and enlarged ovaries (10 and 19 ml) with cysts. An ovarian biopsy revealed normal follicular reserve for age, absence of corpora lutea or albicans and many large antral follicles in one of the ovaries. The second and third families families had only one 46,XY phenotypically female patient due LCH. Direct sequencing of PCR products of the entire exons 5, 6, 7, 8 and 11 of the LHR gene did not reveal mutations in families 1 and 2. We found a heterozygous nonsense mutation of the LHR gene in exon 11 (Arg554→Stop Codon) in the 46,XY patient with Leydig cell hypoplasia and her mother from Family 3 by restriction digest with Taql. This finding was confirmed by direct sequencing. Remaining exons of LHR are being studied in these 3 families, since potential molecular defects could be located in the extensive extracellular domain of LHR. We conclude that clinical manifestations of the LHR abnormalities in genetic females include normal breast development, a relatively hypoplastic uterus, amenorrhea, infertility, elevated gonadotropins, and enlarged cystic ovaries. Clinical manifestations of the LHR abnormalities in genetic males include female external genitalia or micropenis and primary hypogonadism. Restriction digest with Taql is a simple and valuable screening for the 554 nonsense mutation of the LHR gene in patients with LH resistance. Since LCH is autosomal recessive disorder, the heterozygous nonsense mutation in family 3 indicate that the affected patient is a compound heterozygous.