Purpose: To compare the effects of sprint interval training (SIT) and high-intensity interval training (HIIT) on endurance performance and endurance performance determinants. Methods: Twenty-three healthy, trained female subjects were randomly assigned to either 10x30-second SIT (30 sec of running at “all-out” intensity separated by 3.5 min of active recovery) or 4x4-minute HIIT (4 min of running at 90-95% HRmax followed by 3 min of active recovery at 70% HRmax). Both protocols were performed 3 d·wk-1 for 8 wk. Results: HIIT resulted in significantly larger improvement in absolute VO2max, compared to SIT (3.6% vs -0.5%, respectively). This was accompanied by a larger improvement in vVO2max (P < 0.01) and a lower HR at submaximal velocities in HIIT compared to SIT. Only HIIT improved vLT (P < 0.001) and O2 pulse (P < 0.01) and tended to improve more than SIT (both P < 0.06). Both groups improved RE and 3000m running performance with no difference between groups. Only SIT improved anaerobic capacity (9.5%), had a higher session RPE-score (P < 0.001) and improved 300m performance more compared to HIIT (6.2% vs 2.2%, respectively). Of the 17 subjects allocated to SIT, 6 dropped out due to injuries related to the protocol. Conclusion: 4x4-minute HIIT is significantly more effective to improve VO2max compared to 10x30-second SIT in trained females. As SIT resulted in a high injury rate and a higher perceived exertion, a running SIT-protocol is not recommended unless the goal is to improve anaerobic capacity and sprint performance.