Life-history models suggest that ‘costs of reproduction’ can influence the evolution of sexual dimorphism, but empirical data on this effect are scarce. We tested the idea using ‘flying lizards’ (Asian agamids of the genus Draco), because the evolution of ‘flight’ (gliding) is likely to have increased the degree to which pregnancy affects maternal locomotor ability (and hence, we infer, has increased the ‘costs of reproduction’). As predicted, Draco display patterns of sexual dimorphism that are different from those seen in most other lizards. The ‘wings’ (gliding membranes) of female D. melanopogon are significantly larger than those of males of the same body size, a dimorphism that we attribute to natural selection on the ability to glide while encumbered with eggs. Comparisons with non-flying lizards suggest that the same selective pressure has also reversed pre-existing patterns of sexual dimorphism in body size and body proportions (relative head size, relative tail length). Measurements of wing-loading, centre of balance and gliding distances of preserved specimens support the hypothesis that a larger body, relatively larger head, and longer tail may improve a female's aerial mobility when she is gravid. Thus, selection for locomotor ability in egg-burdened females (reduction in ‘costs of reproduction’) may have been responsible for a reversal of sexually-selected traits in this lineage.