A review of literature has been presented involving clinicomorphological characteristics of certain variants of histiocytosis X (HX) and current conception considering this disease as an immunopathologic process that is underlain by reactive proliferation of a specific subtype of antigen-presenting macrophages--Langerhans' cells. Clinico-morphological picture has been described and the results of successful treatment with roferon (human gene-engineering alpha 2-interferon) in 3 adult patients with HX have been presented. It is stressed that the actual effectiveness of the treatment with roferon could not be critically evaluated due to a high frequency of spontaneous remissions of this disease.