The corneal endothelial cells of 25 patients with unilateral acute glaucoma were photographed with a clinical specular microscope. Photography was performed as soon as (average 6-12 h) the IOP had been lowered with iv Acetazolamide and/or Mannitol and topical Pilocarpine therapy, and the corneas had become clear. Peripheral iridectomy was performed on the affected eye and prophylactically on the fellow eye in most of these patients. The follow-up endothelial photographs were taken 6-24 months postoperatively. High intraocular pressure lasting 3 days or more lowered the central endothelial cell density. But a rise in pressure lasting from only a few hours to 2 days did not affect the endothelial cell count. Operative glaucoma procedures caused a loss of central endothelial cells of approximately 4.8% in the series. There was a clear correlation between the duration of elevated pressure and the number of central corneal endothelial cells lost.