BackgroundOverweight and obesity are common health problems which increase the risk of developing several serious health conditions. The main difficulty in the management of weight-loss lies in its maintenance, once it is achieved. The aim of this study was to investigate whether a motivational intervention, together with current clinical practice, was more efficient than a traditional intervention, in the treatment of overweight and obesity and whether this intervention reduces cardiovascular risk factors associated with overweight and obesity.MethodsMulti-centre cluster randomized trial with a 24-month follow-up included 864 overweight/obese patients randomly assigned. Motivational intervention group (400 patients), delivered by a nurse trained by an expert psychologist, in 32 sessions, 1 to 12 fortnightly, and 13 to 32, monthly, on top of their standard programmed diet and exercise. The control group (446 patients), received the usual follow-up.ResultsWeight reduction was statistically significant in the second year with a mean reduction of 1.0 Kg in the control group and 2.5 Kg in the intervention group (p = 0. 02). While 18.1% of patients in the control group reduced their weight by more than 5%, this percentage rose to 26.9% in the intervention group, which is statistically significant (p = 0.04). Patients in the motivational intervention group had significantly greater improvements in triglycerides and APOB/APOA1ratio.ConclusionsThe results highlight the importance of the group motivational interview in the treatment of overweight /obese patients in primary care, and in the improvement of their associated cardiovascular risks factors.Trial registration ClinicalTrials.gov Identifier: NCT01006213 October 30, 2009.Electronic supplementary materialThe online version of this article (doi:10.1186/s12875-017-0644-y) contains supplementary material, which is available to authorized users.