Ovarian dysgenesis (ODG1), also known as FSH-resistant ovaries (FSH-RO), is an autosomal recessive gonadal disorder presenting with hypergonadotrophic ovarian failure causing primary amenorrhea and infertility in females, and with small testes and low sperm counts in males. The basic defect of ODG1 (in Finnish families) is a mutation in the FSRH gene, encoding the follicle stimulating hormone receptor expressed in the granulosa cells of ovaries and Sertoli cells in testes. Mutations in FSHR may cause Ovarian hyperstimulation syndrome (OMIM#608115). Female ODG1 patients with a homozygous FSHR mutation have normal sex organ development but primary amenorrhea and infertility. Their FSH levels are markedly elevated while estrogen is low. Heterozygote females show no abnormalities. Homozygote males have sligthly reduced testis size and a low sperm count, indicating a role for FSH in spermatogenesis. Fertility of homozygote males seems to be reduced, particularly at older ages. The diagnosis of ODG1 by clinical and hormonal studies can be confirmed with a gene test.