The assessment of the adenosine deaminase activity (ADA) in the pleural effusion is used for the diagnosis of tuberculous pleural effusion (TPE). To examine whether the procedure can be applied to immunocompromised patients, we analyzed the ADA activity in the pleural fluid of renal transplant recipients. We studied 23 renal transplant patients with TPE (21 men and 2 women; the mean age, 33 years). They were treated at the Yonsei University Hospital between January 1985 and December 2001. Patients with granuloma in the pleural biopsy specimen or positive for Mycobacterium tuberculosis in the pleural fluid culture were recruited. The ADA activity in the pleural effusion of 23 renal transplant patients with TPE was compared with 23 immunocompetent patients with TPE. The mean ADA activity was 69.5 ± 4.6 in renal transplant patients and 65.0 ± 4.9 U/L in immunocompetent patients. Applying the 40 U/L cut-off point, the positivity of ADA was 91.3% in renal transplant patients, and 86.9% in immunocompetent patients. We thus concluded that the measurement of ADA in the pleural fluid is a useful means in the diagnosis of TPE in renal transplant patients.