The aims of the present study were firstly to examine the reproducibility of outdoor flat and uphill cycling time trials (TT), and secondly to assess the relationship between peak power output (W_peak) obtained in the laboratory and outdoor cycling performance in moderately trained cyclists. Eight competitive male cyclists first performed a progressive cycle ergometer test in the laboratory to determine W_peak (W). Thereafter, they performed three 36 km TT (TT₃₆) on a flat course on separate days and at the same time of the day. On a different day, they also performed three 1.4 km uphill TT (TT_1.4) in a single day. The coefficient of variation (CV) values across three TT₃₆ and TT_1.4 ranged from 1.1 - 1.4% and 2.6 - 2.9%, for performance time (min) and mean power (W), respectively. The correlation between absolute W_peak (W) obtained in the laboratory and mean power during TT₃₆ and TT_1.4 was 0.90 (p < 0.01) and 0.98 (p < 0.01), respectively. Absolute W_peak (W) correlated significantly with performance time in TT₃₆ (r = -0.72, p < 0.05) but not in TT_1.4 (r = -0.52, p > 0.05). The correlation between relative W_peak (W·kg^-1) and performance time in TT₃₆ and TT_1.4 was r = -0.65 (p > 0.05) and r = -0.91 (p < 0.01), respectively. In conclusion, under stable environmental conditions, performance time and mean power are highly reproducible in moderately trained cyclists during outdoor cycling TT. Laboratory determined absolute W_peak (W) may predict cycling performance on a flat course but relative W_peak (W·kg^-1) is a better predictor of performance during uphill cycling.

• Under stable environmental conditions, performance time and mean power are highly reproducible in moderately trained cyclists during outdoor flat and uphill cycling time trials.

• Laboratory determined peak power output (W) (W) may predict cycling performance on a flat course.

• Laboratory determined relative W (W·kg) is a better predictor of performance during uphill cycling