There are conflicting reports in the literature which imply that the decrement in maximal aerobic power experienced by a sea-level (SL) resident sojourning at high altitude (HA) is either smaller or larger for the more aerobically "fit" person. In the present study, data collected during several investigations conducted at an altitude of 4300 m were analyzed to determine if the level of aerobic fitness influenced the decrement in maximal oxygen uptake (VO2max) at HA. The VO2max of 51 male SL residents was measured at an altitude of 50 m and again at 4300 m. The subjects' ages, heights, and weights (mean +/- SE) were 22 +/- 1 yr, 177 +/- 7 cm and 78 +/- 2 kg, respectively. The subjects' VO2max ranged from 36 to 60 ml X kg -1 X min -1 (mean +/- SE = 48 +/- 1) and the individual values were normally distributed within this range. Likewise, the decrement in VO2max at HA was normally distributed from 3 ml X kg-1 X min-1 (9% VO2max at SL) to 29 ml X kg-1 X min-1 (54% VO2max at SL), and averaged 13 +/- 1 ml X kg-1 X min-1 (27 +/- 1% VO2max at SL). The linear correlation coefficient between aerobic fitness and the magnitude of the decrement in VO2max at HA expressed in absolute terms was r = 0.56, or expressed as % VO2max at SL was r = 0.30; both were statistically significant (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)