This study reports the incidence of decompression sickness (DCS) occurring in U.S. Navy altitude chambers in association with physiological training of aircrews for the 7-year period from 1 October 1981 to 30 September 1988. There were 140 total cases of DCS in 136,696 chamber exposure, an incidence of 0.10%. Among trainees, there were 78 cases in 111,674 exposures, and incidence of 0.07%. Among inside observers, there were 62 cases in 25,022 exposures and incidence of 0.25%. The incidence of DCS among inside observers was reduced almost two-fold over the last Navy report and is the lowest reported since 1973. Reasons for the reduction are discussed. In addition, analysis of the data includes types of DCS, symptoms, frequency of joint involvement, predisposing factors, altitude and time of onset of DCS, and treatment tables employed.