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Objective. To compare the relative efficacy and tolerability of amitriptyline, cyclobenzaprine, and placebo in the treatment of fibromyalgia, and to identify predictors of response to amitriptyline and cyclobenzaprine.Methods. Two hundred eight patients who fulfilled the American College of Rheumatology criteria for the classification of fibromyalgia were entered into a 6-month prospective, double-blind, multicenter trial and were randomized to 1 of 3 treatment groups: amitriptyline, cyclobenzaprine, or placebo.Results. After 1 month, 21%, 12%, and 0% of the amitriptyline, cyclobenzaprine, and placebo patients, respectively, had significant clinical improvement (amitriptyline versus placebo P = 0.002, cyclobenzaprine versus placebo P = 0.02, amitriptyline versus cyclobenzaprine P not significant). These percentages increased to 36%, 33%, and 19%, respectively, at the 6-month assessment (P not significant). The nature and frequency of side effects reported by patients treated with amitriptyline and those reported by patients treated with cyclobenzaprine were similar. A normal Minnesota Multiphasic Personality Inventory (MMPI) profile at baseline was predictive of clinical improvement at the 1-month evaluation (odds ratio 3.3, 95% confidence interval 1.2-9.0). However, neither the MMPI profile nor any of the demographic, clinical, or functional parameters evaluated at baseline predicted long-term response.Conclusion. Our data confirm the short-term efficacy of amitriptyline and cyclobenzaprine in a small percentage of patients with fibromyalgia. Long-term efficacy could not be demonstrated because of a higherthan-expected placebo response. Predictors of response to these drugs could not be determined.Fibromyalgia is a common condition of unknown etiology, characterized by generalized musculoskeletal pain in association with the presence of multiple tender points at characteristic locations (1,2). Theories about the pathophysiology of fibromyalgia include the interplay of mechanical stresses in the cervical and lumbar spine (1,3), sleep disturbances (1,4,5), psychological factors (H), muscle deconditioning (9), and abnormalities in the function of neurotransmitters (10). Long-term followup of patients