The gonadal function of patients with Turner syndrome (TS) is variable. Individuals with mosaicism characterized by 45,X/46,XX or 45,X/47,XXX are more likely to experience spontaneous menarche compared with other karyotypes. Prepubertal gonadotropins of TS patients with spontaneous menarche are reportedly normal or significantly lower than those of patients with induced menarche. The present study investigated an index of spontaneous and cyclical menstruation at 10-12 years old in TS. Subjects comprised 50 patients with TS, divided into three groups: Group A (n=7), with spontaneous menarche before 16 years old and regular menstruation for at least 1 year and 6 months; Group B (n=6), with irregular menstruation since menarche leading to secondary amenorrhea despite spontaneous menarche before 16 years old; and Group C (n=37), without spontaneous breast budding before 14 years old or without spontaneous menarche before 16 years old. Karyotype, LH and FSH concentrations at 10 and 12 years old were analyzed retrospectively. Spontaneous and cyclical menstruation was more frequently observed in TS with mosaicism characterized by 45,X/46,XX or 45,X/47,XXX than in TS with other karyotypes, as previously described. Spontaneous and cyclical menstruation in TS was observed when serum FSH level was <10 mIU/mL at 12 years old, suggesting this FSH level as an index of spontaneous and cyclical menstruation in TS.