Objectives: This study examined the time course for minimal clinically significant improvement in pain severity during the initial 12 weeks of treatment in patients with fibromyalgia taking duloxetine. Methods: Four double-blind, placebo-controlled trials of duloxetine were pooled. Patients received duloxetine 60 mg/d, 120 mg/d, or placebo. Clinically significant treatment response (≥30% reduction in pain severity on the 24-hour average pain severity of the Brief Pain Inventory scale) was assessed over 12 weeks. Results: At endpoint, 46.9% of duloxetine 60-mg-, 48.6% of duloxetine 120-mg-, and 32.1% of placebo-treated patients (P<0.001 for both doses) had ≥30% improvement on average pain from baseline. The probabilities of achieving ≥30% response at Weeks 1, 2, 4, 8, and 12 among duloxetine 60-mg-treated patients were 27%, 44%, 45%, 47%, and 49%, respectively, and among duloxetine 120-mg-treated patients were 35%, 43%, 53%, 53%, and 51%, respectively (P<0.01 vs. placebo at each week, for both doses). Among patients who did not respond by Weeks 1, 2, 4, and 8, the percentages of duloxetine 60-mg-treated patients who achieved a response by the endpoint of the study were 36.9%, 29.8%, 28.9%, and 26.9%, respectively. Discussion: This article examines the time course for minimal clinically significant improvement in pain severity in duloxetine-treated patients with fibromyalgia. It provides information about continued treatment in patients who do not initially respond to duloxetine. This information could potentially help physicians facing clinical decisions about management of fibromyalgia with duloxetine. © 2010 Lippincott Williams & Wilkins.