This study determined whether a 4-wk high-intensity interval training program (HIT) would improve the 40-km time trial performances (TT40) of 8 competitive cyclists (peak O2 uptake 5.2 +/- 0.4 I.min-1) with a background of moderate-intensity endurance training (BASE). Before intervention, all cyclists were tested on at least three separate occasions to ensure that their baseline performances were stable. In these tests, peak sustained power output (PPO) was measured during a progressive exercise test, muscular resistance to fatigue was determined during a timed ride to exhaustion at 150% of PPO (TF150), and a TT40 was performed on a cycle-simulator. The coefficient of variation for all baseline tests was < 1.7 +/- 1.3% (mean +/- SD). Cyclists then replaced 15 +/- 2% of their approximately 300 km.wk-1 BASE training with HIT, which took place on 6 d and consisted of six to eight 5-min repetitions at 80% of PPO, with 60-s recovery between work bouts. HIT significantly improved TT40 (56.4 +/- 3.6 vs 54.4 +/- 3.2 min; P < 0.0001), PPO (416 +/- 32 vs 434 +/- 34 W; P < 0.01) and TF150 (60.5 +/- 9.3 vs 72.5 +/- 7.6 s; P < 0.01). The faster TT40 was due to a significant increase in both the cyclists' absolute (301 +/- 42 vs 326 +/- 43 W; P < 0.0001) and relative (72.1 +/- 5.6 vs 75.0 +/- 6.8% of PPO; P < 0.05) power output after HIT. These results indicate that a 4-wk program of HIT increased the PPO and fatigue resistance of competitive cyclists and improved their 40-km time trial performances.