Relative size at onset of maturity (RSOM) is defined as size at first reproduction divided by asymptotic maximal size. RSOM is remarkably constant among species within many higher clades of animals, but varies widely among tree species from the Pasoh Forest Reserve, Malaysia according to the work of S. C. Thomas. RSOM was examined for 16 mid-storey and canopy tree species from a second tropical forest at Barro Colorado Island (BCI), Panama. Interspecific variation in RSOM was equally large for BCI and Pasoh and was unrelated to gap dependence or life form for BCI species. The shape of the relationship between size and the proportion of individuals that were reproductive differed between forests, with an abrupt increase over a narrow range of sizes at Pasoh and a more gradual increase over a wider range of sizes at BCI. Both overtopping trees and heavy liana infestation reduced the probability that BCI trees were reproductive. This presumably reflects reduced availability of carbon for reproduction. We speculate that greater liana loads and a greater abundance of large, shade-casting trees at BCI may increase variation among individuals and contribute to the relatively wide range of sizes characterized by a mixture of sterile and fertile individuals observed for most BCI species.