BACKGROUND During SCUBA-diving, relative changes of the pressure in the diving mask, compared to the environmental pressure, are transmitted to the eye and to the periocular tissue. Barotrauma results from lack of pressure equilibration. MATERIAL AND METHODS In 15 divers (6 experienced, 9 beginners) the pressure difference (delta-p) between inside and outside of the diving mask was measured every second during recreational SCUBA-diving. Data were transmitted wireless to a modified standard diving computer. An overall dive time of 323 minutes was analysed. RESULTS Mean delta-p was 14.8 mbar (-44 to , std.dev. +/- 9), it was not dependent on the diving depth (r2 = 0.0004). Delta-p oscillated between 0 and 25 mbar (0-19 mm Hg), parallel to respiration. Negative delta-p values were 9.5 times more frequent in beginners than in experienced divers. Negative pressure peaks (changes > or = 10 mbar for more than 6 sec) occurred in the beginner group exclusively (p = 0.01). CONCLUSIONS During SCUBA diving, ocular tissues undergo oscillating changes of environmental pressure, parallel to respiration. This may be important in eyes with previous surgery. Pressure peaks, leading to severe ocular barotrauma, are easily avoidable.