The question of how group therapy format may affect therapeutic outcomes and processes has been posed over the past 50 years, but trials are heterogeneous and robust conclusions cannot be made. Systematic research on the differential effectiveness of open versus closed psychotherapy groups is scarce. On the basis of empirical findings to date, no global superiority of either open or closed group therapy exists. Open and closed group therapies are equally effective, presumably because of different effect factors. An indication for open or closed group therapy must be made according to clinical requirements on a case-by-case basis.