Pentoxifylline, an analogue of the methylxanthine theobromine, inhibits the proliferation and certain biosynthetic activities of fibroblasts derived from normal human skin. Fibroblasts from the skin of patients with keloids, scleroderma and morphoea were cultured in vitro in the presence and absence of pentoxifylline (100-1000 micrograms/ml) to determine whether it inhibits fibroblast proliferation and the production of collagen, glycosaminoglycans (GAG), fibronectin and collagenase activity. The exposure of subconfluent fibroblast cultures to pentoxifylline resulted in non-lethal, dose-dependent reductions in serum-driven fibroblast proliferation, with 1000 micrograms/ml pentoxifylline virtually negating the proliferative effect of serum on the cells. The fibroblasts assayed as confluent cultures produced reduced amounts, by up to 95%, of collagen and GAG, dependent on the concentration of pentoxifylline, both in the presence and absence of serum. Pentoxifylline similarly inhibited the fibronectin production by keloid and scleroderma fibroblasts, but had no effect on collagenase activity.