A heparin-surface-modified PMMA IOL has recently been developed in order to give the IOL a hydrophilic surface and to reduce postoperative inflammatory reactions in the eye. It is known that severe inflammation accompanies a tremendous increase in leucocytes in IOLs. A foreign-body reaction is characterized by the formation of histiocytic giant cells. The topical administration of corticosteroids, or the surface passivation of IOLs, cannot prevent a foreign-body reaction. Specular microscopy was used for in vivo investigation of cellular reaction in IOL surface of heparin-surface-modified lenses versus regular PMMA lenses. The double-masked and randomized study included 53 patients investigated 1 week and 1 month postoperatively. Small, round, and spindle-shaped cells were the cells most frequently observed after 1 week. These cells were identified in all cases. They were found in a higher density on the surface of the control lenses (43 cells/mm2) than on the heparin-surface-modified lenses (25 cells/mm2). Giant cells were found in two patients after 1 week; both of these patients belonged to the control group. Small, round, and spindle-shaped cells were also seen in all patients 1 month postoperatively. Again, they were found in a higher frequency on the control lenses (38 cells/mm2) than on the heparin-surface-modified lenses (13 cells/mm2). Giant cells were found in 18 patients 1 month postoperatively. They were found exclusively on the control lenses; the difference was significant (p less than 0.0001).