The purpose of this study is to determine whether there are some differences in the treatment responses to a multidisciplinary fibromyalgia (FM) treatment related with the baseline body mass index (BMI) of the participants. Inclusion criteria consisted of female sex, a diagnosis of FM (American College of Rheumatology criteria), age between 18 and 60 years, and between 3 and 8 years of schooling. Baseline BMI was determined, and patients were randomly assigned to one of the two treatment conditions: conventional pharmacologic treatment or multidisciplinary treatment. Outcome measures were pain intensity, functionality, catastrophizing, psychological distress, health-related quality of life, and sleep disturbances. One hundred thirty patients participated in the study. No statistical significant differences regarding pre-treatment outcomes were found among the different BMI subgroups, and between the two experimental conditions for each BMI category. General linear model analysis showed a significant interaction group treatment × time in pain intensity (p < .01), functionality (p < .0001), catastrophizing (p < .01), psychological distress (p < .0001), sleep index problems (p < .0001), and health-related quality of life (p < .05). No significant interactions were found in BMI × time, and in BMI × group treatment × time. There are not differences among normal weight, overweight and obese patients with FM regarding their response to a multidisciplinary treatment programme for FM which combines pharmacological treatment, education, physical therapy and cognitive behavioural therapy.