The primary objectives of this study were to assess the efficacy and safety of duloxetine for reducing pain severity in fibromyalgia patients with or without current major depressive disorder. This was a 6-month, multicenter, randomized, double-blind, placebo-controlled study. In total, 520 patients meeting American College of Rheumatology criteria for fibromyalgia were randomly assigned to duloxetine (20 mg/day, 60 mg/day, or 120 mg/day) or placebo, administered once daily, for 6 months (after 3 months, the duloxetine 20-mg/day group titrated to 60 mg/day). The co-primary outcome measures were the Brief Pain Inventory (BPI) average pain severity score and Patient Global Impressions of Improvement (PGI-I) score. Safety was assessed via treatment-emergent adverse events, and changes in vital sign, laboratory, and ECG measures. Compared with placebo-treated patients, those patients treated with duloxetine 120 mg/day improved significantly more on the co-primary outcome measures at 3 months (change in BPI score [−2.31 vs −1.39, P < 0.001] and PGI-I [2.89 vs 3.39, P = 0.004]) and at 6 months (change in BPI [−2.26 vs −1.43, P = 0.003] and PGI-I [2.93 vs 3.37, P = 0.012]). Compared with placebo, treatment with duloxetine 60 mg/day also significantly improved the co-primary measures at 3 months and BPI at 6 months. Duloxetine was efficacious in patients both with and without major depressive disorder. There were no clinically significant differences between treatment groups in changes in vital signs, laboratory measures, or ECG measures. Study results demonstrated that duloxetine at doses of 60 mg/day and 120 mg/day appears to be safe and efficacious in patients with fibromyalgia.