Objective. To assess the one-year effectiveness on weight loss of a 3-week balneotherapy program (BT). Method. A Zelen double consent randomised controlled trial to compare one-year BMI loss between a 3-week BT program versus usual care (UC) for overweight or obese patients (BMI: 27-35 kg/m(2)), associated or not with a dietary motivational interview (DMI) during the follow-up, using a 2 × 2 factorial design. Main analysis was a per protocol analysis comparing patients attending BT to patients managed by UC, matched on sex, overweight or obese status, DMI randomisation and a propensity score to attend BT or to be managed by UC. Results. From the 257 patients who completed the follow-up, 70 patients of each group could be matched. Mean BMI loss was 1.91 kg/m(2) [95%CI: 1.46; 2.35] for the BT patients and 0.20 kg/m(2) [-0.24; 0.64] for the UC patients (P < 0.001), corresponding to a significant BT benefit of 1.71 kg/m(2) [1.08; 2.33]. There was no significant effect of DMI and no interaction with BT or UC. No adverse reaction was observed for patients attending BT. Conclusion. A 3-week BT program provided a significant one-year benefit over the usual GP dietary advice for overweight and obese patients.