Pharmacologic treatment remains the primary therapeutic approach in fibromyalgia management, but different non-pharmacologic measures, especially physical therapies and psychologically-based interventions, have also shown to be effective in the treatment of this disease. The objective of the present randomized controlled trial is to compare the efficacy and tolerability of two different modalities of active low-impact exercise in warm water: stretching and Ai Chi. Each group of patients will receive 18 physiotherapy sessions lasting 60 minutes and will be evaluated at baseline, at treatment termination, and after 4 and 12 weeks of follow up. Main outcome measures are the Fibromyalgia Impact Questionnaire (FIQ) and the Pittsburgh Sleep Quality Index (PSQI). Secondary outcome measures include the Beck Depression Inventory (BDI), the State and Trait Anxiety Inventory (STAI), and the SF-12 Health Survey (SF-12. Data analysis will be done using repeated measures ANOVA, unpaired Student's t test, and effect sizes' estimation (ES).