OBJECTIVE To investigate the effect of selective laser trabeculoplasty (SLT) on the intraocular pressure (IOP) of untreated fellow eyes in patients with open-angle glaucoma. METHODS Retrospective chart review. METHODS Charts of all patients who underwent SLT at the University of Texas Southwestern Medical Center at Dallas between September 2003 and May 2006 were reviewed. Each patient had IOP measurements by Goldmann applanation tonometry in both eyes preoperatively, and at 1 hour, 2 weeks, 3 months, and 6 months postoperatively. Patient age, gender, diagnosis, central corneal thickness (CCT), previous intraocular surgeries, and degrees of laser treatment were tabulated for each patient. Patients with a history of previous glaucoma surgery in either eye were excluded as were those who underwent any change in glaucoma medications or further laser or surgical intervention in either eye within 6 months of SLT. Data were analyzed using a paired two-tailed t-test, an unpaired two-tailed t-test, ANOVA, and linear regression. RESULTS A total of 43 patients were included through 6 months of follow-up. Mean reduction in IOP in the treated eye was 3.9 +/- 0.6 mmHg or 18.8% (p < 0.001) at final exam. Mean IOP reduction in the fellow untreated eye was 2.1 +/- 0.5 mmHg or 11.2% (p < 0.01). Patients with higher preoperative IOPs had a greater reduction in IOP in both eyes (p < 0.001 for treated eyes, and p = 0.02 for untreated eyes). Patients who were on a larger number of glaucoma medications preoperatively had a greater response in both eyes (treated eye p = 0.002, untreated eye p = 0.008). There was no significant difference in IOP response in either eye based on age, gender, CCT, degrees of treatment, or phakic status. CONCLUSIONS SLT produces a sustained and statistically significant IOP reduction in the fellow untreated eyes of patients with open-angle glaucoma. The results of our study support a biological mechanism of action for SLT. Limitations of this study include its retrospective design, relatively small sample size, a possible effect of increased compliance with medical therapy following SLT, and an inherent bias of excluding patients who underwent a change in medications or further laser or surgical therapy during the period under review.