Context: Little information exists regarding FSH values in very young girls with Turner syndrome (TS). Objectives: The objective of the study was to evaluate the pattern, natural progression, and karyotype-related differences in FSH secretion in young, prepubertal girls with TS. Study Design: FSH was measured at study entry and annually for 2 yr. Setting: The Toddler Turner study was conducted at 11 U.S. pediatric endocrine centers. Study Participants: Eighty-eight girls with karyotype-proven TS aged 9 months to 4 yr participated in the study. Main Outcome Measures: By-karyotype differences in FSH concentration and age-related changes in FSH were measured. Results: Mean (± SD) FSH was markedly elevated in the 45,X (n = 56: 68.3 ± 36.0 IU/liter) and Other groups [n = 15 (excluding three subjects with Y-containing karyotypes): 52.7 ± 50.8 IU/liter] but was minimally elevated in girls with 45,X/46,XX mosaicism (n = 14: 10.1 ± 13.5 IU/liter, P < 0.005 both comparisons). Over the 2-yr period, FSH declined in the 45,X group (-13.4 IU/liter·yr, P < 0.0001). Nonetheless, only three of 159 FSH values fell within normal range for age at any time during the 2-yr study. FSH decline was similar in the Other group (-14.3 IU/liter·yr, P = 0.0032). In contrast, no significant decrease in FSH with age was observed in the 45,X/46,XX group. Conclusions: In contrast to the original report of FSH concentrations in individuals with TS, this study demonstrates distinct differences in patterns of FSH secretion between young girls with monosomy TS, who have persistent elevation of FSH to age 6 yr, and those with 45,X/46,XX mosaicism, whose FSH values suggest retained ovarian function in the majority. These findings have implications for patient management and family counseling.