Results of trabeculectomy (TE) and viscocanalostomy (VCO) were compared in a prospective randomised study in two fellow eyes of 22 consecutive patients with bilateral symmetrical high-tension glaucoma. Rates of overall surgical success with intraocular pressures (IOP) < or = 18 mm Hg with or without medications were 91 for the TE, and 95 for the VCO group after a mean follow-up of 18 months. Complete success rates without medications were 64 and 59 for TE and VCO groups, respectively (p = 0.750). Both procedures significantly reduced IOP, however, IOP course following trabeculectomy was significantly lower (p = 0.026). Rates of complications were not found to be different between the two groups of eyes, except for an apparent--though not significant (p = 0.066)--increase in cataract progression with TE. Various types of conjunctival blebs were detected in all eyes with surgical success in both groups, however, diffuse, elevated or multi-cystic functional blebs appeared to be more predominant in eyes with TE, compared to the VCO group in which low-lying, localised blebs had a higher incidence (p = 0.015). Viscocanalostomy was found to be a safe and effective filtration technique in patients with uncomplicated high-tension glaucoma, though IOP decrease was more pronounced with trabeculectomy.