After glaucoma filtering surgery, an excessive repopulation of Tenon's capsule fibroblasts (TCFs) with the accumulation of extracellular matrix including collagen types during wound healing may cause scarring of the bleb, resulting in surgical failure. Pentoxifylline (PTX) and pentifylline (PTF), both methylxanthine derivatives, are known to inhibit protein synthesis and proliferation of cells in vitro. We examined the effects of these agents on the proliferation of cultured human TCFs and their production of type-I collagen COOH-terminal peptide (PIP) and laminin to investigate the potential use of the agents as antifibrotic agents after filtering surgery. Both agents inhibited the proliferation of cultured human TCFs and their production of PIP and laminin. The inhibitory effects of PTF on proliferation and production of PIP and laminin were greater than those of PTX. These methylxanthine derivatives may have clinical utility in preventing excessive bleb scarring after glaucoma filtering surgery.