The immune responses against pulmonary tuberculosis are still poorly defined. This study describes changes in leucocyte and lymphocyte subsets during treatment to find reliable immunological markers for the disease and treatment response. Flow cytometric peripheral blood immune phenotyping, routine haematology and sputum microbiology were performed on 21 HIV-negative adult tuberculosis (TB) patients with positive sputum cultures during therapy in comparison with 14 healthy purified protein derivative (PPD)-positive volunteers. Patients at diagnosis showed high absolute neutrophil and monocyte counts which fell during treatment but low lymphocyte subset counts which increased [except natural killer (NK) and NK T cells]. High counts of a population of CD3(dim)/CD56+ NK T cells at diagnosis correlated significantly with negative sputum culture after 8 weeks of treatment. A multivariate classification technique showed improved correlation when NK cells were taken into account. In conclusion, peripheral blood white cell counts change significantly during treatment and counts at diagnosis, especially CD3(dim)/CD56+ NK T cells, hold promise in predictive models of TB treatment response.