OBJECTIVE: Assess the efficacy of duloxetine 60/120 mg (N = 162) once daily compared with placebo (N = 168) in the treatment of patients with fibromyalgia, during six months of treatment. METHODS: This was a phase-III, randomized, double-blind, placebo-controlled, parallel-group study assessing the efficacy and safety of duloxetine. RESULTS: There were no significant differences between treatment groups on the co-primary efficacy outcome measures, change in the Brief Pain Inventory (BPI) average pain severity from baseline to endpoint (P = 0.053) and the Patient's Global Impressions of Improvement (PGI-I) at endpoint (P = 0.073). Duloxetine-treated patients improved significantly more than placebo-treated patients on the Fibromyalgia Impact Questionnaire pain score, BPI least pain score and average interference score, Clinical Global Impressions of Severity scale, area under the curve of pain relief, Multidimensional Fatigue Inventory mental fatigue dimension, Beck Depression Inventory-II total score, and 36-item Short Form Health Survey mental component summary and mental health score. Nausea was the most common treatment-emergent adverse event in the duloxetine group. Overall discontinuation rates were similar between groups. CONCLUSIONS: Although duloxetine 60/120 mg/day failed to demonstrate significant improvement over placebo on the co-primary outcome measures, in this supportive study, duloxetine demonstrated significant improvement compared with placebo on numerous secondary measures.