A transitory deposit of a fibrin-like material in the anterior chamber following extracapsular cataract extraction and intraocular lens (IOL) implantation is described. In two studies, one retrospective of 352 operations and one prospective of 189 operations, the fibrinoid reaction was observed in 17% and 11% of the eyes, respectively. The reaction appeared in the early postoperative period in an otherwise quiet eye and the signs varied from a few threads in the pupil area to a dense pupillary membrane in front of the IOL. The deposit disappeared one day to three weeks postoperatively, usually without any remnants. The majority of eyes with the fibrinoid reaction had received a posterior chamber IOL. A strong association with the exfoliation syndrome was found and an increased vascular permeability is suggested as a probable cause of the reaction.