During a nine-year period, 14 cases of mycobacterial infection (tuberculosis) developed in 403 renal transplant recipients at the King Faisal Specialist Hospital and Research Centre in Riyadh, Saudi Arabia, an incidence of 3.5 per cent. The annual incidence of tuberculosis was about 50 times higher than that in the general population. Infection was disseminated in nine (64.3 per cent), pulmonary in four (28.6 per cent), and genitourinary in 1 (7.1 per cent). In one patient tuberculosis was transmitted by the donor's kidney. The clinical manifestations were often ill-defined and not different from that in the normal host. Cultures from all patients grew Mycobacterium tuberculosis; concomitant infection with other organisms was present in five patients (35.7 per cent). Two of 18 patients (group 1) with positive pretransplant tuberculin skin test developed tuberculosis after transplantation (11 per cent), and neither received isoniazid prophylaxis; three of 70 patients (group 2) with negative skin tests developed tuberculosis after transplantation (4.3 per cent). The difference between the two groups was not statistically significant. Review of all published cases of mycobacterial infections in renal transplant recipients revealed 130 cases. Tuberculosis was disseminated in 38.7 per cent, pulmonary in 40.2 per cent, cutaneous in 12 per cent, and miscellaneous in 9.4 per cent. Atypical mycobacteria were responsible for 29 per cent of disseminated infections, 8 per cent of pulmonary infections and all cases of cutaneous and articular tuberculosis. Invasive procedures were needed to establish the diagnosis in 21 of 33 disseminated cases but in only three of 47 cases of pulmonary tuberculosis (p less than 0.0001). The mortality rate from disseminated disease was 37 per cent and from all other forms of tuberculosis was 11 per cent (p less than 0.005). These findings (1) confirm the higher incidence of tuberculosis in renal transplant recipients, compared to the general population; (2) suggest that pretransplant skin testing probably has little value in identifying patients at risk; (3) show that disseminated tuberculosis is common after renal transplantation and requires invasive procedures for diagnosis; (4) confirm that the donor kidney may be an important source of infection; and (5) indicate that concomitant infection with other organisms is common.