Men generally have an inherent performance advantage over women due to their average greater height and muscle mass and power, as the result of correspondingly different exposures to androgens. Therefore, it is considered fair that in sports men and women compete in separate categories. The question now emerging is whether reassigned transsexuals can compete in fairness with others of their new sex. The pertinent question is how far the previous effects of testosterone in male-to-female transsexuals (M-F) are reversible upon androgen deprivation so that M-F have no advantage over women, and, vice versa, what the effects are of androgen exposure in female-to-male transsexuals (F-M) on variables relevant to competition in sports. Before puberty, boys and girls do not differ in height, muscle and bone mass. Recent information shows convincingly that actual levels of circulating testosterone determine largely muscle mass and strength, though with considerable interindividual diversity. This study analyzed the effects of androgen deprivation in 19 M-F and of androgen administration to 17 F-M on muscle mass, hemoglobin (Hb) and insulin-like growth factor-1 (IGF-1). Before cross-sex hormone administration, there was a considerable overlap in muscle mass between M-F and F-M. In both M-F and F-M, height was a strong predictor of muscle mass. Androgen deprivation of M-F decreased muscle mass, increasing the overlap with untreated F-M, but mean muscle mass remained significantly higher in M-F than in F-M. Androgen administration to F-M increased muscle mass without inducing an advantage over nontreated M-F. The conclusion is that androgen deprivation in M-F increases the overlap in muscle mass with women but does not reverse it, statistically. The question of whether reassigned M-F can fairly compete with women depends on what degree of arbitrariness one wishes to accept, keeping in mind, for instance, that similar blood testosterone levels in men have profoundly different biologic effects on muscle properties, rendering competition in sports intrinsically a matter of how nature endows individuals for this competition.