In two patients with a Molteno implant, the filtration membrane was excised 2 and 21 months postoperatively because of uncontrolled elevation of intraocular pressure. This tissue was examined by light and electron microscopy and the findings compared with those of a third functioning Molteno filtration membrane obtained from an enucleation specimen. The most conspicuous differences were the high density and cellularity of the connective tissue of the non-functioning membranes and the frequent cell pyknosis and necrosis in the functioning membrane. In addition, one of the non-functioning membranes showed a condensed cellular layer on the inner surface. There were no inflammatory cell infiltrates in any of the three specimens. We conclude that the variable results of Molteno implants are related to the quantities of tissue components of the filtration membrane.