We examined a total of 13 xanthogranulomas in order to examine the mechanism by which these lesions grow. Six xanthogranulomas from children < 2 years of age were compared with lesions from seven patients > 10 years of age. All lesions were stained with antibodies directed against Ki-67 and bcl-2 protein. There were no significant differences between the two groups. There was a slight trend toward higher rates of proliferating histiocytes within lesions from children (p < 0.18). It is unclear from our data whether this higher rate is correlated more with the age of the patient or with the duration of the lesion. Inhibition of cellular apoptosis, as demonstrated by expression of bcl-2, appears to play a minor role in the growth of xanthogranulomas in either children or adults. Xanthogranulomas appear to be tumors that grow by proliferation of "histiocytes," independent of the age of onset.