The recent finding that a mutation in the FSH receptor gene causes ovarian dysgenesis prompted the present study to determine the phenotype caused by this mutation. Twenty-two patients with ovarian dysgenesis and a 566C-->T mutation in the FSH receptor gene (designated FSH-resistant ovaries or FSHRO) were compared with 30 clinically similar patients with ovarian dysgenesis (designated ODG) who did not have this mutation. The genealogical studies suggested a founder effect of the FSH receptor gene mutation in Finland. Clinically, both groups of patients were characterized by primary or early secondary amenorrhea, variable development of secondary sex characteristics, and high serum levels of FSH and LH. Notable differences were observed in median adult height (FSHRO patients were shorter) and the occurrence of follicles judged by transvaginal sonography (observed in 6 of 8 FSHRO vs. 1 of 11 ODG) and ovarian histology (present in all 9 FSHRO vs. 1 of 4 ODG). These findings suggest that a subset of ovarian dysgenesis patients with the FSH receptor mutation 566C-->T is pathogenetically distinct, possibly due to residual receptor activity, and that these patients can be tentatively identified by demonstrating the presence of ovarian follicles and confirmed by mutation analysis.