Menstrual-cycle profiles of salivary progesterone concentration, obtained by radioimmunoassay of daily samples collected throughout the cycle, were obtained from Thai (n = 232) and British (n = 130) adolescent girls up to 4 years postmenarche. These profiles were graded from 1 to 5 ranging, respectively from concentrations at the detection limit of the assay to profiles generally observed for the mature premenopausal woman. Contingency table analysis of the grade frequencies for Thai-British pairs of girls matched for chronological age and age at menarche (n = 2 x 90) demonstrated that British girls had more mature cycles (22/90) than Thais (11/90) (P less than 0.05) particularly in the first 2 years postmenarche (P less than 0.01). For these matched pairs of girls there was no evidence to support the view that girls with an early age of menarche develop their profiles more quickly following menarche than those with a late age of menarche, as previously reported and which was thought to be important in the development of breast cancer. The findings of this study also suggest that adolescent girls in Britain develop their menstrual cycle profiles of salivary progesterone more quickly than their Thai counterparts and this may be of value in formulating hypotheses regarding any role that ovarian progesterone secretion may have on subsequent breast cancer risk.