A prospective, randomised comparison of Nd:YAG laser iridotomy and operative peripheral iridectomy in the fellow eye of 52 patients presenting with acute angle closure glaucoma has been undertaken. Nd:YAG laser iridotomy resulted in a significantly higher incidence of iris bleeding at the time of treatment (p less than 0.05), higher intraocular pressure one hour (p less than 0.02) and three hours (p less than 0.02) following treatment, and a higher incidence of focal corneal oedema 24 hours following treatment (p less than 0.001). One laser treated patient developed acute angle closure glaucoma two weeks following treatment despite the presence of a small patent iridotomy. After a mean follow-up period of 11.8 months there was no significant difference between the two groups in visual acuity, or intraocular pressure. It is concluded that Nd:YAG laser iridotomy is a satisfactory alternative to operative peripheral iridectomy in the prophylactic treatment of fellow eyes.