Background: Cancer treatment outcome may be impaired due to treatment-related adverse effects like decreased cardiorespiratory fitness. Evidence on exercise during or after chemotherapy shows positive effects on cardiorespiratory fitness, fatigue and quality of life (QoL) in cancer patients. However, optimal timing of starting exercise is unknown. This study aimed to investigate if an exercise intervention that starts during chemotherapy (early group) is superior to a program starting after completion of chemotherapy (late group) to reduce cardiovascular morbidity. Methods: In this multicenter randomized controlled trial, 266 patients (testicular-, (n = 95), breast-, (n = 139), and colon cancer (n = 30) or non-Hodgkin lymphoma (NHL) (n = 2)), treated with curative chemotherapy were randomized to a 24 week aerobic and resistance exercise intervention starting either early, i.e. during chemotherapy (n = 131) or late, i.e. at completion of chemotherapy (n = 135) (NCT01642680). Effect on VO2 peak was evaluated with intention-to-treat linear mixed-effect models, adjusted for baseline values (T0) and diagnosis at post-chemotherapy (T1), post-exercise intervention (T2) and 1-year post-exercise intervention (T3, i.e., primary endpoint). Here we report T0, T1 and T2 data. Secondary endpoints were QoL (EORTC-QLQ-C30) and fatigue (MFI-20), with higher scores indicating more fatigue. Results: Median age was 33 yrs for testicular-, 52 yrs for breast- and 64 yrs for colon cancer and NHL patients. Patients in the early group declined significantly less in VO2 peak and QoL at T1 compared to the late group (adjusted between-group differences were 3.2 ml/min/kg (95% confidence interval CI 2.3 to 4.1, P < 0.0001) and 5.8 (95% CI 0.6 to 10.9, P = 0.028). Patients in the early group experienced reduced general and physical fatigue at T1 (adjusted between-group differences were -2.0 (95% CI -3.3 to -0.8, P = 0.002) and -2.9 (95% CI -4.3 to -1.5, P < 0.0001). At T2, VO2 peak, QoL, general and physical fatigue were comparable and regained baseline levels (adjusted between-group differences - 0.08 ml/min/kg (P = 0.9), -1.4 (P = 0.7), 0.7 (P = 0.3) and 0.2 (P = 0.7), respectively. Conclusions: A supervised exercise program for patients with testicular-, breast- and colon cancer that is initiated at start of curative chemotherapy effectively reduces a decline in VO2 peak and QoL and reduces fatigue. After completion of the exercise intervention, initiated both during and after chemotherapy, patients regained their baseline VO2 peak, levels of fatigue and QoL. Clinical trial information: NCT01642680.