This study investigated the utility of a verification phase for increasing confidence that a "true" maximal oxygen uptake had been elicited in 16 male distance runners (mean age (+/-SD), 38.7 (+/- 7.5 y)) during an incremental treadmill running test continued to volitional exhaustion. After the incremental test subjects performed a 10 min recovery walk and a verification phase performed to volitional exhaustion at a running speed 0.5 km.h(-1) higher than that attained during the last completed stage of the incremental phase. Verification criteria were a verification phase peak oxygen uptake <or=2% higher than the incremental phase value and peak heart rate values within 2 beats.min(-1) of each other. Of the 32 tests, 26 satisfied the oxygen uptake verification criterion and 23 satisfied the heart rate verification criterion. Peak heart rate was lower (p = 0.001) during the verification phase than during the incremental phase, suggesting that the verification protocol was inadequate in eliciting maximal values in some runners. This was further supported by the fact that 7 tests exhibited peak oxygen uptake values over 100 mL.min(-1) (>or= 3%) lower than the peak values attained in the incremental phase. Further research is required to improve the verification procedure before its utility can be confirmed.