Three cases of multicentric reticulohistiocytosis showing typical clinical, histologic, and ultrastructural findings are reported. In one, gastric cancer occurred; in the other two cases, severe polyarthritis was the only detectable internal involvement. The serine proteinases. urokinase and tissue-type plasminogen activator, were evaluated both with the autohistographic technique and spectrophotometric assay in lesional skin and synovia. Urokinase levels appeared grossly increased in the lesional synovia and moderately increased in the lesional skin. We suggest that urokinase, presumably released by the activated proliferating histiocytes, may play a major role in the extracellular matrix degradation leading to erosion of cartilage and adjacent bone in multicentric reticulohistiocytosis.