Neodymium:YAG (Nd:YAG) and argon laser iridotomies were compared in a prospective, randomized clinical trial of 43 patients with bilateral chronic pupillary-block glaucoma. All patients had one eye randomly assigned to argon and the fellow eye assigned to Nd:YAG laser treatment. Follow-up ranged from 20 to 42 months. Iridotomy closure was not observed in Nd:YAG-treated eyes, but nine (21%) argon iridotomies required retreatment. Visual loss due to progression of laser-induced lens or corneal damage was not observed in any eye. Nine (21%) argon-treated eyes and eight (19%) Nd:YAG-treated eyes required laser trabeculoplasty for further intraocular pressure (IOP) lowering after iridotomy. Five (12%) argon-treated and two (5%) Nd:YAG-treated eyes required intraocular filtration surgery for long-term IOP control, but this difference was not statistically significant. There were no significant long-term differences between these treatment modalities.