The proliferating cells in fibromatoses are myofibroblasts that produce abundant stromal collagen and contain intracellular native and widely spaced collagen fibers. To assess the clinical and cellular effects of colchicine in such tumors, this drug was administered to three patients, one with musculoaponeurotic desmoid fibromatosis, one with Dupuytren's palmar fibromatosis, and one with Peyronie's disease. All three patients had an excellent clinical response, with reduction of tumor size and improvement of contracture. Two cases were studied ultrastructurally; the main cellular changes detected were collapse of the rough endoplasmic reticulum cisternae, reduction of myofilaments, and disappearance of intracellular widely spaced collagen. The findings from this study indicate another probable application for colchicine and support the concept that collagen fibers can be formed intracellularly.