The purposes of this study were firstly to determine the relationship between the peak power output (Wpeak) and maximal oxygen uptake (VO2max) attained during a laboratory cycling test to exhaustion, and secondly to assess the relationship between Wpeak and times in a 20-km cycling trial. One hundred trained cyclists (54 men, 46 women) participated in the first part of this investigation. Each cyclist performed a minimum of one maximal test during which Wmax and VO2max were determined. For the second part of the study 19 cyclists completed a maximal test for the determination of Wpeak, and also a 20-km cycling time trial. Highly significant relationships were obtained between Wpeak and VO2max (r = 0.97, P less than 0.0001) and between Wpeak and 20-km cycle time (r = -0.91, P less than 0.001). Thus, Wpeak explained 94% of the variance in measured VO2max and 82% of the variability in cycle time over 20 km. We concluded that for trained cyclists, the VO2max can be accurately predicted from Wpeak, and that Wpeak is a valid predictor of 20-km cycle time.