We determined immune function parameters in 41 newly diagnosed patients with chronic lymphocytic leukaemia (CLL) and correlated these findings with the clinical data and the subsequent course of the disease. The ratio of helper to suppressor T cells (CD4/CD8), the proportion of circulating natural killer (NK) cells and the NK activity were significantly low in clinical stage B and C patients. Among patients presenting with advanced disease, those who subsequently had a more severe course, characterised mainly by frequent respiratory infections, were found to have at presentation a significantly lower CD4/CD8 ratio (x +/- SEM = 0.95 +/- 0.09, vs 1.28 +/- 0.14), a very low proportion of NK cells (4.78 +/- 0.85, vs 11.75 +/- 2.1%) and decreased amount of gamma-globulins (0.66 +/- 0.08, vs 0.97 +/- 0.09 g/dl), in comparison with patients with a much milder later course. These simple parameters of immune function seem to have prognostic value for patients with CLL.