A previously healthy 6-year-old boy complained of persistent pain of about 1 month's duration in the left lower chest wall. He had no history of fever, weight loss, or any other systemic symptoms, and had continued his usual physical activities. Physical examination was normal, apart from tenderness of the left lower rib cage on palpation. There was no soft tissue swelling or discoloration of the overlying skin. Complete blood count and chest radiography were normal. Magnetic resonance imaging of the chest showed expansion of the tip of the 12th left rib and abnormally high signal intensity in the marrow on T2-weighted pulse sequences, indicative of edema. Substantial soft tissue edema was also seen around the affected bone. Excision of the distal segment of the 12th rib showed that the expanded marrow cavity was occupied by friable tissue. Histological examination showed that the lesion was formed by lymphocytes, plasma cells, eosinophils, and histiocytes, which had abundant eosinophilic cytoplasm, and large nuclei with a coffee bean configuration characteristic of Langerhans' cells. With immunoperoxidase the histiocytes showed strong cytoplasmic membrane positivity with CD1a antibody and cytoplasmic positivity with S-100 protein antibody. These findings confirmed the diagnosis of Langerhans' cell histiocytosis. [ABSTRACT FROM AUTHOR]