Menstrual dysfunction is common among athletes with very low body mass, such as long distance runners and dancers, and is usually associated with hypothalamic dysfunction. The purpose of this study was to investigate the menstrual status of swimmers, in whom exercise is nonweight bearing and thinness is, thus, not essential. Questionnaires recording the menstrual history of 69 young competitive swimmers (aged 16.4 +/- 0.5) were compared to those of 279 age-matched controls. Age of menarche (M) was significantly (P < 0.005) delayed among swimmers (13.8 +/- 0.2 yr) compared to controls (13.0 +/- 0.1 yr). Eighty-two percent of swimmers had menstrual irregularities after M compared to 40% of control, with longer duration of these irregularities (16 vs. 4 months; P < 0.005). A subset of 24 swimmers was studied further for body composition, pubertal stage, and reproductive hormone levels. Estradiol levels were normal in all post-M swimmers (273 +/- 20 pmol/L) and higher than average in pre-M (383 +/- 44 pmol/L). FSH levels were normal in all subjects (10.7 +/- 1.6 IU/L), LH was mildly elevated (17.1 +/- 1.2 IU/L), and the LH/FSH ratio was 1.7. Levels of dehydroepiandrostenedione sulfate and androstenedione, but not testosterone, were higher than average in all groups of swimmers. The results of this study indicate that female competitive swimmers are vulnerable to delayed puberty and menstrual irregularities, but the associated hormonal profile is very different from the hypothalamic amenorrhea described in dancers and runners. We, therefore, suggest a different mechanism for reproductive dysfunction in swimmers that is associated not with hypoestrogenism, but, rather, with mild hyperandrogenism. A distinction among the various types of athletic amenorrhea should be made based on hormonal profiles with attention to their weight and somatotype.