Mycobacterial infection is a serious opportunistic infection in renal transplant recipients. The incidence is higher in developing than in developed Western countries. This study is a single-centre retrospective review of the records of 2502 renal transplant recipients in Belgium. Fourteen cases of mycobacterial infection (9 Mycobacterium tuberculosis and 5 atypical mycobacterial infection) were diagnosed. The time interval between transplantation and diagnosis was 64 +/- 80 months (mean +/- SD, range 5-188) for M. tuberculosis and 92 +/- 75 months (range 14-209) for atypical mycobacterial infection. The localisation of M. tuberculosis was pulmonary/pleural in 67% and extrapulmonary in 33%. The atypical mycobacterial infections were located in skin, tendons, and joints. Eight patients received IV prednisolone pulse therapy for acute rejection long before the time of mycobacterial infection. The initial antimycobacterial therapy consisted of a combination of isoniazid, rifampicin, and ethambutol in all patients. In patients with M. tuberculosis infection, a good response to antimycobacterial therapy was obtained. In patients with atypical mycobacterial infection, initial treatment was successful in 3 out of 5 patients, in 1 patient recurrence was diagnosed and in another patient, who is still under treatment at present, the initial treatment was adjusted after identification of the atypical mycobacterium and its antibiogram. The incidence of mycobacterial infection after renal transplantation did not increase with newer immunosuppressive therapy. The major risk factor is the total dose of corticosteroids. All patients responded well without major reductions in immunosuppressive therapy. Chemoprophylaxis for high-risk patients still is recommended.