The case of a two-year-old girl with generalized histiocytosis, probably induced by phenobarbital, is reported. Symptoms, including intermittent fever, systemic lymphadenopathy, maculopapular skin eruption and hepatosplenomegaly, suggested Langerhans cell histiocytosis. Laboratory examinations revealed leukocytosis with lymphocytosis and eosinophilia and a high LDH serum level, while GOT and GPT were within normal ranges. Cytological studies of lymph node and pleural effusion specimens revealed proliferation and infiltration of Langerhans cell histiocytes with eosinophilia. No histiocyte proliferation was observed in the bone marrow or skin. The clinical manifestations shown by the patient were, however, transient, and improved spontaneously after the discontinuation of phenobarbital. The case was considered to be one of phenobarbital hypersensitivity syndrome based on clinical course and laboratory findings. The mechanism and differential diagnosis of the syndrome are discussed.