Segment reassortment of negative strand viruses is an important mechanism for the development of new virus strains with altered pathogenicity. This study reports on in vitro generation of reassortants between Andes (ANDV) and Sin Nombre (SNV) viruses. Although they both cause hantavirus pulmonary syndrome (HPS), ANDV is the only hantavirus that has been transmitted from person to person (). Following dual infection of cells with ANDV and SNV, 8.9% of 337 progeny plaques contained reassortants, of which 66% were diploid, and 34% were monoploid. The monoploid reassortants contained the S and L segments of SNV and ANDV M segment. Analysis of replication of the monoploid reassortant indicated its efficiency was similar to ANDV rather than SNV. Results described in this study illustrate the ability to rapidly generate new hantavirus genotypes between genetically unrelated viruses by gene reassortment and provide a tool to dissect the pathogenesis of these important viruses.