It is well known that transplant recipients exhibit an increased rate of cancer in comparison with the general population. This increased risk is commonly attributed to an impaired surveillance of the recipient´s immune system against cancer cells due to the effect of immunosuppressive drugs. Because the occurrence of tumors relatively rare, a quantitation of tumor risk is difficult and requires large patient cohorts for analysis. It is of interest to know which types of tumors are increased more than others and whether genetic (ethnic) or geographical differences exist. In terms of prospective intervention, it is very important to establish whether particular drug treatment regimens might be associated with higher rates of tumor occurrence than others. Recent claims that certain immunosuppressive drugs may even be tumor-protective require careful examination. The Collaborative Transplant Study (CTS) initiated a special effort in 1984 to document posttransplant tumors soon after their clinical diagnosis. Because the problem of underreporting of tumor data was recognized early on, a strong effort was made to establish a high-quality database in which the accuracy and completeness of tumor data was certified yearly by participating investigators. Currently, the CTS database for the analysis of posttransplant tumors contains data on more than 250.000 patients with a combined follow up of more than one million patient years. This wealth of information will form the basis for an analysis on the incidence of posttransplant tumors to be presented at this year´s MESOT congress in Ankara.