The purposes of this study were firstly to determine the relationship between the peak power output (W _peak) and maximal oxygen uptake (VO_2max) attained during a laboratory cycling test to exhaustion, and secondly to assess the relationship betweenW _peak 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 whichW _max andVO_2max were determined. For the second part of the study 19 cyclists completed a maximal test for the determination ofW _peak, and also a 20-km cycling time trial. Highly significant relationships were obtained betweenW _peak andVO_2max (r=0.97,P<0.0001) and betweenW _peak and 20-km cycle time (r= –0.91,P<0.001). Thus,W _peak explained 94% of the variance in measuredVO_2max and 82% of the variability in cycle time over 20 km. We concluded that for trained cyclists, theVO_2max can be accurately predicted fromW _peak, and thatW _peak is a valid predictor of 20-km cycle time.