Twenty-four professional divers were put through a series of electroencephalographic (EEG) and psychometric tests at the surface and at 180 m after a fast compression (15 min) in an attempt to determine their susceptibility to high-pressure nervous syndrome (HPNS). The subjects were classified according to the EEG changes between the surface and 180 m: group 0, less than 10% increase in theta-band activity in the anterior region of the scalp; group 1, between 10 and 100% increase in theta-activity; group 2, theta-activity increase above 100%. Eight subjects were selected to make a dive to 450 m to verify the quality of our classification (3 from group 0, 3 from group 1, 2 from group 2). The results showed that, for the psychometric tests, the large individual variability between the surface and 180 m does not allow us to prejudge the behavior of each subject at 450 m, but the classification established from absolute values is stable from the surface to 450 m. For the EEG activities, intensity of the modifications at 450 m can be predicted at group level by a 180-m dive, performed with fast compression and similar breathing mixtures.