OBJECTIVE: To evaluate the effect of swimming on pain, functional capacity, aerobic capacity, and quality of life in patients with fibromyalgia (FM). DESIGN: Randomized controlled trial. SETTING: Rheumatology outpatient clinics of a university hospital. PARTICIPANTS: Women with FM (N=75; age range, 18-60y) randomly assigned to a swimming group (SG) (n=39) or a walking group (WG) (n=36). INTERVENTION: The SG performed 50 minutes of swimming 3 times a week for 12 weeks, with a heart rate at 11 beats under the anaerobic threshold. The WG performed walking with a heart rate at the anaerobic threshold, with the same duration and frequency as the SG. MAIN OUTCOME MEASURES: Participants were evaluated before the exercise protocols (t0), at 6 weeks (t6), and at 12 weeks (t12) after the onset of the protocols. The primary outcome measure was the visual analog scale for pain. The secondary measurements were the Fibromyalgia Impact Questionnaire and the Medical Outcomes Study 36-Item Short-Form Health Survey for quality of life; a spiroergometric test for cardiorespiratory variables; and the timed Up & Go test for functional performance. RESULTS: Patients in both groups experienced improvement in pain after the 12-week program, with no difference between groups (P=.658). The same results were found regarding functional capacity and quality of life. Moreover, no statistical difference between groups was found regarding aerobic capacity over time. CONCLUSIONS: Swimming, like walking, is an effective method for reducing pain and improving both functional capacity and quality of life in patients with FM.