A case is presented of a female infant with an atypical histiocytoma. A gradually enlarging brown lesion was noted on the left side of the chest at the age of 2 weeks. Microscopic study of a biopsy revealed an ill-defined infiltration of spindle cells with indented nuclei. The tumor cells were positive for CD14, HLA-DR, lysozyme, alpha-1-antitrypsin and alpha-1-antichymotrypsin, and negative for CD1, CD3, CD8, CD10, CD19, CD68 and S-100 by immunohistochemistry. Electron microscopy demonstrated no distinct Birbeck's granules, but aberrant granules were seen in a small number of cells. At 7 months of age, a nodule with similar histologic features was noted in the nuchal region, but was incompletely resected. The patient remains recurrence-free at 36 months of age. This case is thought to be a benign form of non-X histiocytoma.