STAGER, J. M. AND L. K. HATLER. Menarche in athletes: the influence of genetics and prepubertal training. Med. Sci. Sports Exerc, Vol. 20, No. 4, pp. 369-373, 1988. It is unclear whether the later menarche observed in competitive athletes is due to genetic factors or environmental factors related to intense prepubertal training. In order to investigate the importance of these factors upon the age of menarche (AOM), 263 competitive swimmers and 71 women with no athletic experience were asked to complete questionnaires concerning general health and reproductive development. Based upon their responses, similar questionnaires were sent to the sisters of both groups. Complete data were obtained from 140 athlete/sister pairs and 43 control/sister pairs. Results indicate that the athletes were older at menarche than their sisters, the non-athletes, and the sisters of the non-athletes (14.3 yr, 13.7yr, 12.9 yr, and 13.0 yr, respectively). The sisters of the athletes were significantly older at menarche than the controls and their sisters, while the AOM of the non-athletic controls and their sisters did not differ. The interpretation of the data is complicated by the fact that the sisters of the athletes were likely to be athletes (75%), while those of the control group tended to be non-athletic (74%). The athletes differed from their sisters who trained prior to menarche (13.9 years) and from those who did not engage in prepubertal training (13.1 years). Controls did not differ from any of their sister groups, nor did they differ from the untrained sisters of athletes. In terms of the similarity of the AOM within the sister pairs, the athletes/sisters and controls/sisters were equally similar. Significant correlations were observed between athletes and their sisters who trained prior to menarche and controls and their sisters who did not. The data can be interpreted to support the importance of inherited characteristics and intense prepubertal activity upon the AOM of athletes. Because neither factor can absolutely be ruled out, it is proposed that both factors may be involved in the observed later menarche of competitive swimmers. However, a similar conclusion cannot be drawn from athletic populations who do not begin athletic participation until menarche has already been attained. This suggests that the later menarche is a function of factors associated with enhanced or prolonged athletic performance.