Histiocytosis X (HX) follows a variable course from the self-limited eosinophilic granuloma (EG) to the aggressive disseminated disease in infants. Some classifications place EG in a well-differentiated category and the disseminated infantile form in a morphologically atypical group. However, criteria for determining prognosis from biopsy material have not received widespread acceptance. We retrospectively reviewed 51 cases of HX, emphasizing the cytological atypia and mitotic activity of the Langerhans' histiocytes, as well as the presence of other parameters. Ten patients died. We were unable to predict clinical outcome from the histopathological findings. Some cases of EG showed mild atypia and high proliferative rates while some fatal disseminated cases had a bland appearance. The prognosis is best predicted by clinical parameters. The findings favor HX being a reactive, nonneoplastic proliferation of the Langerhans' cells.