The scaling of stem and crown was studied in eight Cecropia species in Amazonian and southeastern forests of Brazil. The Amazonian species, C. concolor, C. palmata, C. purpurascens, C. sciadophylla, and C. ulei were studied in Manaus, Amazonas, Brazil. The southeastern species, C. glaziovii, C. hololeuca, and C. pachystachya were studied in Linhares, Espírito Santo, Brazil. Measures of diameter, height, number of branches, number of leaves, and total leaf area were log transformed and regressed on height. Three models of mechanical designs of trees, elastic, constant stress, and geometric similarity, were tested for Cecropia. None of the models can totally describe Cecropia, but geometric similarity was a close approximation. Most of the species did not vary in diameter–height relationships between unbranched and branched individuals. Safety factors diminished with height in most species studied. The crown–height relationships were similar for all species. Numbers of branches and leaves showed some variation among species and are related to height of first branching. Total leaf area had a constant allometric relationship among species, although regression intercepts differed according to species leaf areas. The scaling relationships of stem and crown in Cecropia varied with adult size of the studied species.