Doppler ultrasound was used in five subjects to detect intracardial gas bubbles during decompressions to altitude. At a simulated altitude of 8,000 m, neither intracardial bubbles nor symptoms of decompression sickness occurred. At 9,000 m, bubbles were registered in two subjects, one of which had questionable bends. At 11,500 m, bubbles were registered in all but one subject and two had bends. The three subjects who had not gotten bends were exposed to an air-breathing period of 30 min or, in one case, even 45 min at 2 ATA, for extra nitrogen loading, followed by decompression to 11,500 m. These subjects had heavy showers of bubbles followed by bends. In all cases with decompression sickness during the decompressions to altitude, intracardial bubbles were registered prior to the appearance of symptoms. The technique may be used in studies of decompression sickness without provoking actual symptoms, thus making the studies safer.