A review of case reports, hypobaric chamber training data, and experimental evidence indicated that the threshold for incidence of Altitude Decompression Sickness (DCS) was influenced by various factors such as prior denitrogenation, exercise or rest and period of exposure, in addition to individual susceptibility. Fitting these data with appropriate statistical models has the potential for estimating the frequency of occurrence of DCS at various altitudes under different experimental conditions and allows us to examine the influence of various factors on the threshold for DCS. This approach was illustrated by logistic regression analysis on the incidence of DCS below 9,144 m (30,000 ft). Estimations using these regressions showed that under a noprebreathe, 6-h exposure, simulated extravehicular activity profile, the threshold for symptoms occurred at approximately 3,353 m (11,000 ft); while under a no-prebreathe, 2-h exposure profile with knee-bends exercise, the threshold occurred at 7,925 m (26,000 ft). These examples showed that definition of threshold altitude should be qualified by the particular combination of experimental variables under which it was observed.