Balneotherapy (BT) is considered an effective, non-pharmacological approach, in the multimodal treatment of the Primary Fibromyalgia Syndrome (FS). However, the evidence of efficacy and tolerability of BT in FS is still limited. This is a prospective multi-center two-cohort observational study. The main aim will be the comparison of the Minimal Clinically Important Difference (according to Fibromyalgia Impact Questionnaire—FIQ...