Forty cases of histiocytosis of Langerhans' cells (26 children and 14 adults) were evaluated; 20 were localized forms (with single or multiple bone involvement); another 20 were generalized forms, with organic dysfunction (lung, liver, hemopoietic system) in 7 children. This feature has a poor prognostic significance, as shown by the fact that 3 of these 7 patients died. In 3 patients single skin involvement was found, with spontaneous recovery one month after the diagnosis in one; in one case the single involvement was lymphadenopathic, associated to Hodgkin's lymphoma, and in another it was hypothalamic. These cases of single involvement have been followed up between 1 and 4 years. In 4 of 7 patients with generalized disease a reduction of OKT8 with an increased OKT4: OKT8 ratio was found, as reported in other cases from the literature. This finding gives support to an immunological rather than neoplastic basis for the disease. In all 40 cases the diagnosis was morphological, with positive staining for S-100 protein in 38.