Objectives: The pineal gland is an important neuroendocrine organ accounting for the melatonin secretion and chronobiology that regulate circadian rhythm. This study was designed to compare pineal gland volume (PGV) with healthy controls and patients with fibromyalgia syndrome (FM), in which sleep quality and efficiency is reduced. Patient and Methods: In this cross-sectional study, PGV and functional pineal gland volume (FPGV) of FM patients with age- and sex-matched healthy controls were compared. All MR imaging studies were performed using a 3 Tesla scanner with a multi-channel phased array head coil. The volume of pineal glands and pineal cysts were calculated from 3D MP RAGE images using the formula: volume= AP x transverse x craniocaudal diameter x 0.523. Results: There was no significant difference in PGV and FPGV between the FM group and healthy controls (p=0.374 and p=0.421, respectively). In the correlation analysis, age was negatively correlated with PGV and FPGV in the FM group (r=-0.496, p=0.010; r=- 0.477, p=0.014, respectively). No significant correlation was detected between age, PGV and FPGV in the control group (r= 0.022, p=0.916; r= -0.019, p=0.925, respectively). Conclusions: Based on the results, there was no significant difference between the FM group and healthy controls regarding PGV and FPGV. However, PGV and FPGV were decreased by advancing age in the FM group in which melatonin therapy is offered as an option. Keywords: Fibromyalgia syndrome, pineal gland volume, functional pineal gland volume, pineal gland magnetic resonance imaging