OBJECTIVE: Meta-analyses show that exercise interventions during cancer treatment reduce cancer-related fatigue. However, little is known about the cost-effectiveness of such interventions. Here we aim to assess the cost-effectiveness of the 18-week physical activity during cancer treatment (PACT) intervention for patients with breast and colon cancer. The PACT trial showed beneficial effects for fatigue and physical fitness. DESIGN: Cost-effectiveness analyses with a 9-month time horizon (18 weeks of intervention and 18 weeks of follow-up) within the randomised controlled multicentre PACT study. SETTING: Outpatient clinics of 7 hospitals in the Netherlands (1 academic and 6 general hospitals) PARTICIPANTS: 204 patients with breast cancer and 33 with colon cancer undergoing adjuvant treatment including chemotherapy. INTERVENTION: Supervised 1-hour aerobic and resistance exercise (twice per week for 18 weeks) or usual care. MAIN OUTCOME MEASURES: Costs, quality-adjusted life years (QALY) and the incremental cost-effectiveness ratio. RESULTS: For colon cancer, the cost-effectiveness analysis showed beneficial effects of the exercise intervention with incremental costs savings of €4321 and QALY improvements of 0.03. 100% of bootstrap simulations indicated that the intervention is dominant (ie, cheaper and more effective). For breast cancer, the results did not indicate that the exercise intervention was cost-effective. Incremental costs were €2912, and the incremental effect was 0.01 QALY. At a Dutch threshold value of €20 000 per QALY, the probability that the intervention is cost-effective was 2%. CONCLUSIONS: Our results suggest that the 18-week exercise programme was cost-effective for colon cancer, but not for breast cancer.