Hundemer GL, Jersey SL, Stuart RP, Butler WP, Pilmanis AA. Altitude decompression sickness incidence among U-2 pilots: 1994-2010. Aviat Space Environ Med 2012; 83:968–74. Introduction: The U-2 aircraft exposes its pilots to cabin pressures equivalent to 29,500 ft (8992 m) during flight, placing them at risk for decompression sickness (DCS). Historical data documenting DCS in the U-2 pilot community is lacking. This study assesses how rates and types of DCS have changed temporally in the U-2 flight program. Methods: We created a database of all DCS cases among U-2 pilots from 1994 through 2010. Cases were analyzed by date of occurrence and symptoms experienced. Flight data were collected to calculate DCS incidence rates. Results: From 1994–2010, there were 73 documented DCS cases in U-2 pilots. Between 1994 and 2005, the number of annual cases ranged from 0-5; between 2006 and 2010, the number of annual cases increased to 6-10. Additionally, there was a trend toward more severe (neurologic and pulmonary) cases between 2006 and 2010 with 22 cases compared to 10 cases the preceding 12 yr. The most common presentations of U-2 DCS were joint pain (59%), mainly involving large joints, and generalized neurologic symptoms (44%). From 2006–2010, there was an increase in the average annual flight hours per pilot to meet wartime operational needs that correlated temporally with the increase in number and severity of DCS cases. The DCS risk per flight was 0.076% from 1994–2005 but increased to 0.23% from 2006–2010. Conclusions: DCS remains prevalent among U-2 pilots. An increase in number and severity of cases correlated temporally with increased operational tempo of the U-2 squadron. Changes in cockpit pressurization and limiting the length and frequency of hypobaric exposure may reduce future incidence.