We conducted a retrospective comparison of the effectiveness of argon laser trabeculoplasty in controlling increased intraocular pressure in two different age groups treated for medically uncontrolled primary open-angle glaucoma. Of 15 eyes of patients less than 40 years old who had primary open-angle glaucoma, nine (60%) had uncontrolled intraocular pressures postoperatively and needed filtering surgery within two years of argon laser trabeculoplasty. Only two of 29 (7%) eyes in older patients had unacceptably high intraocular pressures during a mean (+/- 1 S.D.) follow-up period of 17 +/- 5 months. Older eyes had greater decreases in intraocular pressure (12 +/- 6 mm Hg) than younger eyes (5 +/- 6 mm Hg) after laser treatment. Failure in young eyes appeared to correlate with a high preoperative intraocular pressure. Thus, argon laser trabeculoplasty is not a reliably effective form of therapy for younger patients with primary open-angle glaucoma.