The prostitute, and her sister in sin - the so-called 'fallen' woman - were veritable obsessions of American Progressive Era culture. Their cumulative presence, in scores of controversial theatrical productions, demonstrates the repeated obsession with the prostitute figure in both highbrow and lowbrow entertainments. As the first extended examination of such dramas during the Progressive Era, Sisters in Sin recovers a slice of theatre history in demonstrating that the prostitute was central to American realist theatre. Such plays about prostitutes were so popular that they constituted a forgotten genre - the brothel play. The brothel drama's stunning success reveals much about early twentieth-century American anxieties about sexuality, contagion, eugenics, women's rights and urbanization. Introducing previously unexamined archival documents and unpublished play scripts, this original study argues that the body of the prostitute was a corporeal site upon which modernist desires and cultural imperatives were mapped.