This study was aimed at investigating the immunologic relationship between cytokine production pattern and tuberculin negativity in patients with active Mycobacterium tuberculosis infection. After classifying patients by the extent of pulmonary involvement and the size of the tuberculin reaction, we evaluated the rate of cytokine positivity in peripheral blood to determine whether there is a characteristic cellular immune reaction pattern which could partly explain the tuberculin negativity in some of these cases. The significance of tuberculin anergy occurring in some cases with M. tuberculosis infection is still not clear. We investigated the ratio of IL-4, IL-10, IL-12, CD-4, CD-8 expressing lymphocytes in the peripheral blood of patients with active M. tuberculosis infection and correlated the percentage of the reactive cells with the positivity or negativity of tuberculin skin reactions. Twenty-eight patients were included in the study, with 11 healthy volunteers serving as controls. 10 ml of venous blood was drawn before starting anti-mycobacterial treatment. A tuberculin skin test was performed, introducing intracutaneously 5 TU PPD on the forearm with results evaluated after 72 h. Consistent with the reactivity or non-reactivity of the tuberculin skin test, we found a significantly higher ratio of IL-4 and IL-10 positive lymphocytes and a significantly lower ratio of IL-12 in the peripheral blood of patients with tuberculin anergy than in that of tuberculin positive patients or healthy donors. There was no difference in the ratio of the CD-4 CD-8 positive lymphocytes among the three groups. To evaluate whether the differences could be explained by the degree of pulmonary tubercular involvement, we classified the patients into three groups according to the extent and type of X-ray findings. Seven out of eight tuberculin negative patients were classified as grade III, whereas in the tuberculin positive group only seven out of 20 fell in this category. There was no significant correlation between the radiological grade of the patients and the examined in vitro parameters unless the tuberculin reactivity of each patients was also considered. Tuberculin anergy may reflect an inappropriate immune response to the intracellular pathogen. The high percentage of IL-4 and IL-10 positive lymphocytes together with a low percentage of IL-12 positive lymphocytes in the peripheral blood of anergic patients suggests a Th2 biased immune response during the early course of the disease.