An 82-year-old woman developed acute neuropsychiatric signs (confusion, disorders of speech and vision) together with ataxic gait and left hemiparesis mainly affecting the lower limb. For 10 years she had been known to have chronic lymphatic leukaemia (CLL) which had not hitherto needed treatment. The patient had an exceptionally high leucocyte count (1,300,000/microliters), most of the cells being morphologically atypical lymphocytes. The computed tomogram of the skull showed a marginal zone infarct with a hypodense focus in the area of the right middle and posterior cerebral arteries, extending from the cortical to subcortical zones almost as far as the posterior horn. This was interpreted as a leukostasis syndrome with cerebral infarction due to rapid progression of the CLL. Significant reduction of leucocyte count with considerable improvement in clinical signs was achieved after three doses of vincristine and prednisone together with one cycle of COP (cyclophosphamide, vincristine, prednisone) therapy. Although the leucocyte count is not an important criterion in planning the therapy of CLL, in the event of rapidly increasing leucocytosis it is important to begin cytoreductive treatment in good time so as to avert the leukostasis syndrome.