A prospective study of endothelial cell loss following pars plana vitrectomy and pars plana vitrectomy with lensectomy was performed in 21 eyes. Fifteen diabetic patients had either vitrectomy alone or vitrectomy with lensectomy. Six nondiabetic eyes were also included in this study. The average cell loss was between 2.3% and 3.9% in all cases. This difference was not statistically significant from the preoperative measurements of endothelial cell counts. These data suggest that pars plana vitrectomy does not result in significant endothelial cell loss, especially when eyes remain phakic or when the anterior capsule is maintained until the end of the case.