In the largest compressed air tunneling contract for the construction of the Island Line of the Mass Transit Railway system in Hong Kong, 154,390 man-decompressions occurred, of which 142,140 were after exposures to 1 bar (1.97 ATA, 14.7 psig) or above. The maximum working pressure (MWP) was 3.30 bar (4.26 ATA, 47.9 psig). There were 792 cases of type I and 1 case of type II decompression sickness. The manifestations of the cases were generally similar to those reported elsewhere. Oxygen treatment was given to 9 cases and all were successfully treated with no recurrence of symptoms. Minimum effective pressure treatment on 783 type I cases was successful, with 9.6% requiring two or more recompressions. The pressure required to relieve symptoms was more closely related to the interval between completion of decompression after work and commencement of treatment than to the delay between onset of symptoms and treatment. For every 1-h interval or every 1-h delay, an additional pressure of 0.04 bar (0.04 ATA, 0.58 psi) above MWP was required for pain relief. Step-wise multiple regression analysis showed that the four predictors for pressure of relief and the highest pressure used in recompression, respectively, were, in order of descending importance, maximum working pressure, interval before treatment, bends sequence (the nth attack of bends experienced in the present contract, i.e., the sum of previous attacks and the present attack), and duration of exposure.