Immunosuppressive therapies associated with organ transplantation produce an increased risk of cancer development. Malignancies are increased in transplant recipients because of the impaired immune system. Moreover, experimental data point to a tumor-promoting activity of various immunosuppressive agents. In this study, we compared the effects of 4 immunosuppressive agents with different mechanisms of action (cyclosporine, rapamycin, mycophenolic acid, and leflunomide) on the in vitro growth of various tumor cell lines and umbilical vein endothelial cells. To varying degrees rapamycin (10 ng/mL), mycophenolic acid (300 nmol/L), and leflunomide (30 micromol/L) highly inhibited the growth of human rhabdomyosarcoma, hepatocellular carcinoma, colorectal carcinoma, and endothelial cells. In contrast, cyclosporine (100 ng/mL) did not affect their growth. Our data suggest that regimens containing rapamycin, mycophenolic acid, or leflunomide, which have both immunosuppressive and antitumor activities, should be preferred in transplant recipients to minimize the risk of tumors.