Purpose: To investigate the effect of co-morbid psychiatric disorders on clinical symptoms of fibromyalgia syndrome such as pain intensity, fatigue, sleep disturbances, and patients' global assessment. Materials and Methods: One hundred consecutive female patients who were diagnosed with fibromyalgia syndrome according to American College of Rheumatology (ACR) criteria were included in the study. The patients were asked to assess the intensity of their pain, fatigue and sleep disturbances. They were also requested to fill in Symptom Check List (SCL-90 R), Beck Depression Inventory (BDI), Toronto Alexithymia Scale TAS). The patients were divided into 5 groups using Diagnostic and Statistical Manual of Mental Disorders (DSM IV) criteria. The groups were compared according to symptoms of the, patients. Results: There was no significant difference among the groups in terms of age, disease duration, total tender point counts. Thirty-one patients were diagnosed as having depressive disorders, 24 anxiety disorders, 8 somatoform disorders, 1 impulse control disorder, 36 no psychiatric diagnosis. Pain intensity, sleep disturbance, and fatigue complaints are significantly more severe among patients with somatoform, disorder as compared to patients with no diagnosis. Pain intensity in the anxiety disorder group, and sleep disturbance in the depressive disorder group were also found significantly more severe than in the group with no psychiatric diagnosis. Conclusion: Clinical symptoms were significantly more severe in patients with any psychiatric diagnosis. This indicates that co-morbid psychiatric disorders in patients with fibromyalgia modify the self-assessment of clinical symptoms such as pain, sleep disturbances and fatigue.