Objective. Dietary interventions to improve fibromyalgia (FM) symptoms reported conflicting results. This study aimed to treat FM patients with a gluten-free diet (GFD), alternated with a non-restricted gluten-containing diet, followed by a rechallenge of the GFD. Methods. Twenty postmenopausal women with FM and no history of celiac disease participated. A GFD was assigned for 6 months. This was followed by 3 months of a non-restricted gluten-containing diet and then a new GFD for another 6 months. At each visit, the widespread pain index (WPI) and the symptom severity scale (SS) scores were evaluated. Results. The mean age of the patients enrolled was 53.9±10 years. None of the patients had a diagnosis of irritable bowel disease, although they reported vague gastrointestinal symptoms. After 6 months of a GFD, a statistically significant reduction was observed for the WPI (10.3±1.8 vs 7.7±1.4; p<0.0001) and the SS scale (6.4±1.8 vs 4.15±1.6; p=0.0002). The D percentage reduction of the WPI after 6 months of GFD was -24%±9%, while for the SS scale, it was -36%±21%. The following reintroduction of a gluten-containing diet brought about a statistically significant rise in the absolute SS scale and WPI, as well as a D modification of the WPI (21%±13%) and of the SS scale (74%±90%). The rechallenge of the GFD showed a significant improvement in absolute and D WPI (-24%±7%) and SS (-36%±11%). No modifications to the body mass index were found. Conclusions. A GFD improved FM symptoms evaluated with WPI and SS. This was confirmed for the first time, also with a rechallenge of the GFD that followed a non-restricted gluten-containing diet.