Five cases of mycobacterial disease in renal transplant recipients at the Hospital of the University of Pennsylvania were studied. In addition, another 42 cases reported in the literature were reviewed. Disseminated tuberculosis was common, accounting for 20 of the 47 cases. Infection with nontuberculous mycobacteria was also common, accounting for 16 cases. In two cases, a tuberculin nonreactor developed tuberculosis after receiving a transplant from a donor who was a reactor; these cases may represent reactivation tuberculosis in the transplanted kidney. A number of cases appeared preventable. For example, two patients at the Hospital of the University of Pennsylvania had evidence of previous tuberculosis but had not received isoniazid prophylaxis. On the basis of a survey of 26 transplantation centers, it was estimated that the minimal tuberculosis rate among renal transplant recipients is 480 cases per 100,000, vs. 13.1 per 100,000 in the general population.