In 57 cases of B-type chronic lymphocytic leukaemia (B-CLL), the size of peripheral blood lymphocytes was estimated by means of transmission electron microscopy. The mean lymphocyte diameter (MLD) of 50 cells was correlated with the clinical staging as well as the survival. 30 out of 38 patients found in stages 0, I, and II displayed a normal or increased MLD. Conversely, this value was decreased in 12 out of 17 cases in stages III and IV. MLD of patients in clinical stages III and IV was significantly lower as compared with MLD of patients in stages 0, I, and II (P < 0.001). The actuarial curve of 57 patients showed a roughly estimated median survival probability of 43 months. This was of more than 54 months in patients with normal or increased MLD, but only of 22 months in those with a decreased MLD. The difference between these 2 survival curves was statistically significant (P < 0.01). A reduced peripheral blood lymphocyte size, as estimated in suspension by means of transmission electron microscopy, appears to have a bad prognostic significance.