Hypothalamic-pituitary-gonadotropic activity was investigated in 9 girls with premature thelarche, and compared with that in 9 healthy girls and 6 girls with true precocious puberty. The gonadotropin stimulation test with luteinising hormone-releasing hormone was used. Girls with premature thelarche showed luteinising hormone response resembling that of normal girls, and follicle-stimulating hormone (FSH) response quite similar to that of girls with precocious puberty. This suggests that in premature thelarche there is a partial activation of the diencephalic-hypophyseal-gonadal axis, which affects FSH only. Premature thelarche therefore, should be considered as one of the disorders due to altered sensitivity of the hypothalamic receptors which regulate sex maturation.