Taxonomy of Begonia longifolia Blume (Begoniaceae) and related species. Brittonia 55: 19–29. 2003.—The Begonia longifolia Blume complex is revised and seven species are recognized. Begonia crassirostris Irmsch., B. inflata C. B. Clarke, B. tricornis Ridl., and B. trisulcata (A. DC.) Warb. are synonymized with Begonia longifolia Blume, making this the most widely distributed Begonia species within Asia. Begonia tetragona Irmsch. is synonymized with B. acetosella Craib. Descriptions are provided for Begonia longifolia, B. acetosella, and four closely related and also poorly known species with baccate fruit: B. cristata Warb. ex L. B. Sm. & Wassh., B. renifolia Irmsch., B. sarcocarpa Ridl., and B. turbinata Ridl. Begonia hayatae Gagnep. is also recognized as a member of this complex but is not described here. Evidence for a speciation mechanism within this group is discussed. Begonia acetosella and B. sarcocarpa are lectotypified.