Objective To assess the effects of different immunosuppressive drugs on proliferation and function of regulatory T cells (Tregs). Methods We searched MEDLINE (1966 to November 2009), EMbase (from inception to September 2009), and The Cochrane Library (Issue 4, 2009) for clinical and basic research about the effects of various immunosuppressive drugs on Tregs. Data were extracted and methodological quality was assessed by two independent reviewers. Outcome measures for clinical research included blood Tregs levels, acute rejection episodes, and graft function. Outcome measures for basic research included percentage of Tregs proliferation, function, Tregs phenotype, and evidence for possible mechanisms. We analyzed data qualitatively. Results Forty-two studies, including 19 clinical trials and 23 basic studies, were included. The immunosuppressive drugs studied were calcineurin inhibitors (CNIs), Rapa, anti-metabolism drugs, IL-2 receptor-blocking antibodies, T-cell depleting antibodies, and co-stimulation blockade antibodies. Most of the studies were on Rapa and CNIs. Eight basic studies on Rapa and CNIs showed that Rapa could promote the proliferation and function of Tregs, while CNIs could not. Five clinical trials involving a total of 158 patients showed that patients taking Rapa had higher blood concentration of Tregs than those taking CNIs, but no differences were found in graft function (6-42-month follow-up). Conclusion There is substantial evidence that Rapa favors Tregs survival and function. However, the larger number of the blood Tregs in the patients treated with Rapa does not show any correlation with better graft function. Large-sample and high-quality clinical studies with longer follow-up are needed to thoroughly assess the efficacy of immunosuppressive drugs on Tregs and to reveal whether a relationship exists between Tregs and graft function.