Background. Multidrug-resistant pulmonary tuberculosis (MDRTB), a major problem in developing countries, may result from either insufficiency of host cellular immune response or mycobacterial mechanisms which has been more intensively investigated so far. Objectives: The aim of the study was to investigate natural killer cell activity (NKA) and T lymphocyte subsets in HIV- patients with secondary MDRTB. Methods: 20 male patients with MDRTB (mean age 38 +/- 8 years), 15 nonresistant tuberculosis male patients (NRTB) (mean age 36 +/- 11 years) and 12 healthy male controls (mean age 35 +/- 8 years) were included. The percentages of CD3+, CD4+, CD8+, CD25+, CD11b+ and CD16+56+ cells were measured by flow-cytometric analysis of peripheral blood lymphocytes (PBL). NKA was evaluated using the anticandidal index method. Results: The mean tuberculin response was higher in MDRTB and NRTB patients compared to controls (15.4 +/- 3.8, 15.1 +/- 3.3 and 10.9 +/- 2.8 mm, respectively; p < 0.001). There was no significant correlation between PPD response and PBL subsets or NKA. The percentages of both CD3+ and CD3+CD4+ T lymphocytes were significantly lower in MDRTB (62.4 +/- 12.1 and 33.9 +/- 9.0%) compared to NRTB (70.8 +/- 7.5 and 42.9 +/- 8-6%; p < 0.05). Patients with MDRTB had significantly lower NKA compared to NRTB and controls (30.9 +/- 11.3, 49.7 +/- 15.5 and 40.0 +/- 8.5%, respectively; p < 0.01). Conclusion: This reduction in NKA may suggest a role for impaired NK function in the pathogenesis of MDRTB in HIV- patients.