To identify the number of patients with de novo tumors in organ transplant recipients, we collected the information from eleven hospitals of the Instituto Mexicano del Seguro Social with organ transplant programs. During a period of 9 years, we found only five cases of neoplasia. The median age was 26 years. All tumor occurred in renal transplant recipients. All patients received cyclosporine, azathioprine and steroids. One breast tumor, one renal carcinoma, two Kaposi's sarcoma and one angioleiomyosarcoma were found. The frequency of tumors in these patients is lower than reported by other authors, probably due, in our series to the fact that the patients were younger and the existence of a recently created transplant program with a short period of follow-up.