The aim of this study was to compare the effects of two high-intensity, treadmill interval-training programs on 3000-m and 5000-m running performance. Maximal oxygen uptake (.VO(2max)), the running speed associated with .VO(2max) (v.VO(2max)), the time for which v.VO(2max) can be maintained (T(max)), running economy (RE), ventilatory threshold (VT) and 3000-m and 5000-m running times were determined in 27 well-trained runners. Subjects were then randomly assigned to three groups; (1) 60% T(max), (2) 70% T(max) and (3) control. Subjects in the control group continued their normal training and subjects in the two T(max) groups undertook a 4-week treadmill interval-training program with the intensity set at v.VO(2max) and the interval duration at the assigned T(max). These subjects completed two interval-training sessions per week (60% T(max)=six intervals/session, 70% T(max) group=five intervals/session). Subjects were re-tested on all parameters at the completion of the training program. There was a significant improvement between pre- and post-training values in 3000-m time trial (TT) performance in the 60% T(max) group compared to the 70% T(max) and control groups [mean (SE); 60% T(max)=17.6 (3.5) s, 70% T(max) =6.3 (4.2) s, control=0.5 (7.7) s]. There was no significant effect of the training program on 5000-m TT performance [60% T(max)=25.8 (13.8) s, 70% T(max)=3.7 (11.6) s, control=9.9 (13.1) s]. Although there were no significant improvements in .VO(2max), v.VO(2max) and RE between groups, changes in .VO(2max) and RE were significantly correlated with the improvement in the 3000-m TT. Furthermore, VT and T(max) were significantly higher in the 60% T(max) group post- compared to pre-training. In conclusion, 3000-m running performance can be significantly improved in a group of well-trained runners, using a 4-week treadmill interval training program at v.VO(2max) with interval durations of 60% T(max).