We measured serum levels of adenosine-deaminase (ADA) in 44 tuberculosis patients, 70 patients with lung cancer pre treatment, and 130 normal blood donors. Increased ADA levels were found in 64.4% of the tuberculosis patients, in 95.2% in the case of bacillary tuberculosis, but only in 2.8% in the tumor patients. ADA serum levels were statistically significantly increased in tuberculosis patients when compared to lung cancer patients and controls and did not differ between controls and the tumor group. We conclude that serum ADA is a selective marker of immune stimulation in tuberculosis but not in lung cancer. Serum levels positively correlated to the disease extent in tuberculosis of immunocompetent patients. This marker deserves further investigation with respect to its clinical significance.