Over the past few years, the use of mitomycin C (MMC) to reduce surgical failures following filtering procedures has largely increased. After topical application, significant amounts of MMC can be detected intraocularly. In both rabbit and human eyes, a reduction of the normal and elevated intraocular pressures could be achieved with subconjunctival injections of MMC without filtering procedures. Histopathologic examination of the ciliary body in a human eye shortly after surgery with MMC revealed moderate, diffuse toxic changes of the two layers of the ciliary epithelium. By fluorophotometry, eyes after trabeculectomy with MMC had a decreased production of aqueous humor compared with eyes that underwent trabeculectomies without antimetabolites. Independent studies suggest that the toxicity of MMC for the ciliary epithelium may be a significant factor contributing to postoperative hypotony.