We examined the effects of breath holding against a closed glottis (Valsalva's manoeuvre) on intraocular pressure (IOP) in four groups of volunteer subjects: 37 healthy young control subjects, 10 patients with chronic open-angle glaucoma, 11 age-matched control subjects and 8 glaucoma suspects. IOP was recorded by one person using a Digilab 30R/T Pneuma-Tonometer. Chart recordings of the IOP were measured independently by a second investigator. Recordings were taken before, during and 5 minutes after the seated subject exhaled into an aneroid manometer to a pressure of 25 to 35 cm H2O. There was marked variability in the individual responses to Valsalva's manoeuvre in all four groups, with substantial increases (to +9.5 mm Hg) and decreases (to -4.0 mm Hg) in IOP seen. The mean change in IOP during Valsalva's manoeuvre was a small, statistically insignificant decrease in all four groups. The mean change in IOP following Valsalva's manoeuvre was a larger, but still clinically small, decrease. The clinician should be aware of the individual variability in IOP changes with Valsalva's manoeuvre.