The histological and immunohistochemical findings of 34 biopsy specimens from patients with Langerhans' cell histiocytosis (LCH) are reported, with special emphasis on the findings with CD1a mouse monoclonal antibody (MAb) O10 using paraffin-embedded material. Eighteen patients were treated in an adult hospital (mean age, 26.3 years), and the 16 others were children (mean age, 3 years) from a pediatric center. Specimens included 17 bone, 14 skin, two lung, and one lymph node. Tissue was fixed in formalin or Bouin's, and most bone samples were decalcified in nitric acid. Frozen sections were available for 16 cases and electron microscopy for one. Light microscopy was suggestive of LCH in all cases, characterized by large mononucleated cells with abundant eosinophilic cytoplasm and "coffee bean" nucleus. In 33 of the 34 paraffin-embedded LCH samples, mononucleate cells were stained by MAb O10. As controls, we investigated seven tumors expressing S-100 protein (three nevi, two melanomas, two neurofibromas): all were negative with MAb O10. Five non-Langerhans' cell histiocytoses (three juvenile xanthogranulomas and two Rosai-Dorfman lymphadenopathies) were also negative with MAb O10. The results show that in most cases a definitive diagnosis of LCH can be assessed on paraffin-embedded tissue specimens with the help of immunohistochemistry using MAb O10.