A retrospective study of 245 fellow eyes of patients admitted in primary acute angle-closure glaucoma (ACG) is presented. Twenty-five (10.2%) had acute ACG in the fellow eye on presentation. Of the remaining 220, 33 (15%) subsequently developed acute ACG before prophylactic surgery was performed. Possible predisposing factors were analysed. The main findings were that acute ACG in the fellow eye occurred most frequently in hospital, or during the first month after discharge if prophylactic surgery was not performed. The instillation of mydriatics in the operated eye increased the risk to the fellow eye. Pilocarpine and carbonic anhydrase inhibitors confer the best protection on the fellow eye, but no medical regimen entirely prevented an attack of acute ACG in the fellow eye.