Hydroxyurea is an antimetabolite agent used in the treatment of myeloproliferative disorders and sickle cell anaemia. Although hydroxyurea is relatively well tolerated, adverse effects often involve skin and mucous membrane during long-term therapy. A group of 510 patients affected by chronic myeloid leukaemia from 1977 to 1998 has been considered. Only 158 patients were treated with hydroxyurea and fulfilled inclusion/exclusion criteria of this study. A spectrum of severe cutaneous and mucosal changes (inflammatory and neoplastic) was seen in about 13% of patients (21 patients out of 158) and was studied in detail. Cutaneous and mucosal atrophy were observed in all 21 patients. Skin atrophy was often characterized by numerous telangiectases, especially on legs and on sun-exposed sites (16/21). Cutaneous, mucosal and nail hyperpigmentation was evident, albeit with variable extent, in 10 of the 21 patients. Severe stomatitis and glossitis with flattening of papillae were another common finding. Five patients, who received a particularly long treatment with hydroxyurea, developed squamous-cell neoplasms on sun-exposed sites (both squamous-cell carcinomas and keratoacanthomas). Acral changes were characteristic and constant, including acral erythema (21/21), dermatomyositis-like changes on the dorsa of hands (7/21), ulcers localized on acral areas of legs, on genitalia and oral mucosae (20/21). The frequency and the variety of these muco-cutaneous changes are reported and the mechanisms by which hydroxyurea may induce this muco-cutaneous syndrome-like group of changes, are proposed.