- Objective. To assess urine levels of melatonin measured by 6-sulphatoxymelatonin (aMT6s) in patients with fibromyalgia (FM). Methods. Nocturnal aMT6s urine levels were measured by ELISA, in a sample of urine collected from 10 PM to 7 AM from 39 female patients with FM and 39 age matched healthy female controls. All subjects were interviewed and assessed for nonarticular tenderness, FM symptoms, quality of life, and physical functioning. Results. Nocturnal aMT6s levels of patients with FM were not statistically different from those of controls: 16.7 ± 9.2 vs 16.0 ± l1.3 μg, respectively. No association was observed between aMT6s levels of patients with FM and disease duration, reproductive status, sleep and mood disturbances. Conclusion. Nocturnal urine aMT6s levels were similar in patients with FM and controls. Studies are needed to elucidate the possible role of melatonin in FM and should include larger samples of newly diagnosed untreated patients with FM.