BACKGROUND The effect of positive pressure breathing for altitude protection (PBA) on intraocular pressure was studied; the behavior of intraocular pressure both during and after PBA exposure was of particular interest. METHODS Seven subjects were exposed to PBA of up to 60 mmHg at ground-level. The subjects were seated, and wore an aircrew helmet (HGU-55/P), oro-facial mask (M8U-20/P), thoracic counterpressure garment (CSU-17/P) and an extended coverage G-suit (ATAGS). Before, during and after each exposure, intraocular pressure was measured using a Tono-Pen XL applanation tonometer. RESULTS All 7 subjects completed 10 min of PBA at breathing pressures of 30 and 40 mmHg, and 6 subjects completed 10 min at 50 and 60 mmHg. Mean and SEM increases in intraocular pressure, as compared to pre-exposure baseline measurements, were 7.7 +/- 0.6 mmHg at a breathing pressure of 30 mmHg, 12.0 +/- 0.9 mmHg at 40 mmHg, 18.4 +/- 1.3 mmHg at 50 mmHg and 20.0 +/- 0.6 mmHg at 60 mmHg. The difference between each of these increases was significant (p < 0.05), with the exception of that between 50 and 60 mmHg PBA. CONCLUSIONS Intraocular pressure increases as breathing pressure increases. It is likely that this change in intraocular pressure would provide some protection to the retinal vasculature during PBA. In addition, it is unlikely that the temporary elevation of intraocular pressure following pressure breathing is of medical concern.