Early detection of bubbles may provide clues to the mechanism of their formation, and a knowledge of their extent during a decompression may allow the prevention of decompression sickness. We have used ultrasound imaging to study bubble formation in peripheral tissues. The results suggest that: (a) a threshold supersaturation for bubble formation exists; (b) the earliest bubbles are intravascular; (c) before signs of decompression sickness a substantial accumulation of stationary bubbles occurs. Despite the success of Doppler methods in detecting moving bubbles after decompressions normally considered safe, recent studies have shown that the correlation between number of bubbles detected and symptoms of decompression sickness is often poor. We have used a time integral of the ultrasound images, which avoids laborious image analysis, to follow the extent of both moving and stationary bubbles. Human trials involving a wide variety of decompressions suggest that correct prediction of symptoms is possible.