Three male patients aged 52, 49 and 74 years who were treated with hydroxyurea for chronic myeloid leukaemia (CML) or essential thrombocytosis developed severely painful ulcers on the lower legs and feet after an interval varying from 4 months to 2.5 years. These ulcers only healed after discontinuation of the drug. This is an adverse effect of hydroxyurea that is not infrequently seen but is difficult to recognise. The precise pathogenesis is still unclear but micro-circulatory disorders and a direct cytostatic effect on the epidermal cells probably play a role. Often, the drug has already been used for several years before the ulcers develop. They are resistant to treatment unless the hydroxyurea is stopped. Alternatives to hydroxyurea are imatinib for the treatment of CML and interferon alpha-2a or anagrelide for essential thrombocytosis.