Premature thelarche is a benign condition that affects young girls. In contrast, central precocious puberty is considered a more serious disorder that causes progressive secondary sexual development, accelerated growth and skeletal maturation, early epiphyseal fusion, and short adult stature. Differentiation between these 2 conditions is important, but may be difficult on clinical grounds, since patients with both disorders may present initially as isolated breast development. To examine the potential usefulness of gonadotropin measurements in distinguishing early central precocious puberty from premature thelarche, we measured basal and LHRH-stimulated plasma gonadotropin levels in 58 girls with idiopathic premature breast development. The girls were divided into six clinically distinct groups, based on the severity of clinical presentation, ranging from isolated breast development (group A) to complete secondary sexual development and accelerated growth and skeletal maturation (group F). The mean basal plasma LH levels and the peak LH response to LHRH stimulation were significantly less in girls with isolated thelarche (group A) than in girls with complete sexual development (group F). The mean basal plasma FSH levels did not differ between these groups, but the peak FSH response to LHRH was greater in girls with isolated thelarche than in girls with complete sexual development. Thus, girls with isolated premature thelarche had a FSH-predominant response to LHRH [mean ratio of peak LH to peak FSH, 0.29 +/- 0.10 (+/- SD)], while girls with complete sexual development had a LH-predominant response (peak LH/FSH, 4.16 +/- 1.80). All girls with isolated thelarche had peak LH/FSH ratios less than 1, and all girls with complete sexual development had a ratio greater than 1. Girls with early or intermediate manifestations of central precocious puberty, who had features of puberty in addition to breast development but lacked all of the features of group F, comprised groups B-E. These girls also had intermediate peak LH/FSH ratios, ranging from 0.29 +/- 0.10 (group B) to 3.35 +/- 2.66 (group E). We conclude that girls with early central precocious puberty frequently have LH and FSH responses to LHRH that are indistinguishable from the FSH-predominant responses of girls with isolated thelarche. These data are consistent with the hypothesis that premature thelarche and central precocious puberty may represent different positions along a continuum of hypothalamic LHRH neuron activation.