We counted central corneal endothelial cells and measured corneal thickness in 64 human eyes before and three months after cataract extraction and posterior chamber lens implantation. Cataract surgery was the same in all patients and included capsulorhexis and posterior chamber phacoemulsification of the nucleus. Total phacoemulsification energy and time were recorded. A heparin-surface-modified intraocular lens or a regular poly(methyl methacrylate) lens, both having the same design and a 5.0 mm optic, was implanted in the capsular bag with the aid of sodium hyaluronate (Healon). Mean endothelial cell loss was 4%. However, the change in cell density did not correlate with the total phacoemulsification energy used during surgery. Three months after surgery, the central corneal thickness was the same as it had been preoperatively. The results suggest that phacoemulsification in the posterior chamber is a safe procedure even in cases with a hard nucleus.