In Nepal, there are only two hydrometric stations in two of the five recognized hydrologic regions. Thus, data are available at fewer hydrometric stations than usually accepted as adequate for reliable estimation of regional frequency distributions and index flood relationships, which are two major elements of index flood-based regional flood frequency analysis. This study deals with estimation of regional frequency distributions and index flood relationships in the five hydrologic regions of Nepal, with emphasis on the regions that have inadequate hydrometric stations. Synthetic flood data have been used for supplementing the hydrometric data in the two hydrologic regions. A rainfall-runoff model SimHyd was used for synthetic flood data generation. Return period flood estimates of the derived regional flood-frequency relationships were compared with that of at-site and Water and Energy Commission Secretariat of Nepal (WECS) (a frequently used method in Nepal) methods. The regional relationships were found to have far better estimation capabilities than the WECS method.