OBJECTIVE To investigate changes in physiological and performance variables in triathletes following a 4-wk period of reduced training volume and increased training intensity. METHODS Sixteen moderately trained triathletes were randomly allocated to 2 groups: a control (CON) group, which followed their usual training, or a high-intensity interval training (HIIT) group, which completed 2 HIIT sessions per week during 4 wk of reduced training volume Results: Maximal oxygen consumption (VO2max) increased significantly in the HIIT group (P = .03, d = 0.5) but remained unchanged in the CON group. Cycling power at first and second ventilatory thresholds increased significantly in the HIIT subjects (P = .03, d = 1.0) and was unchanged in the CON participants (P = .57). During the simulated triathlon test, pretest-posttest cycling times and average power were unchanged in both groups (P > .05). No significant interactive effects between groups were observed for running time (P = .50). CONCLUSIONS After a 4-wk HIIT program, VO2max and power at first and second ventilatory thresholds were found to have increased significantly while cycling and running performance were unchanged, despite an overall reduction in training time. In the present study, performance was only shown to improve with usual (high-volume) training. Summarizing, in order to improve running or cycling performances, high-volume training programs are highly recommended.