The clinical variability of chronic lymphocytic leukaemia (CLL) is well known. The short and long term prognosis is usually difficult to predict in the individual case. In the present work the proliferative activity in peripheral CLL cells of 38 patients was analysed by ³H‐thymidine uptake and a proliferative index (PI = labelling index × white blood cell count/l × 10⁻⁹) was calculated. A strong correlation between PI and clinical disease activity was found. Thus 22 out of 23 patients with non‐progressive disease had normal values (PI < 10). 8 out of 8 cases with PI > 20 had a clear disease progression in all blood variables. The PI interval 10–20 comprised both patients with progressive and patients with non‐progressive disease. Variations in PI correlated well to variations in clinical disease activity in 8 patients examined at intervals during long term treatment. The results indicate that determination of PI might be used as therapy guide since it was normalized during successful treatment.