This is the first placebo-controlled trial evaluating the efficacy of the selective serotonin reuptake inhibitor (SSRI), escitalopram, in the treatment of premenstrual dysphoric disorder (PMDD). Women with PMDD (intention-to-treat population, n = 151) were treated intermittently for 3 months, during luteal phases only, with 10 mg/d escitalopram, 20 mg/d escitalopram, or placebo. Escitalopram was found to exert a marked and a dose-dependent symptom-reducing effect, 20 mg/d being clearly superior to 10 mg/d. Although the primary outcome parameter, that is, the sum of the symptoms irritability, depressed mood, tension, and affective lability, was decreased by 90% with 20 mg/d escitalopram, the effect of active treatment on breast tenderness, food craving, and lack of energy was more modest and not significantly different from that of placebo; this outcome supports our previous assumption that the former symptoms are more inclined to respond to intermittent administration of an SSRI than are the latter. Although the placebo response was high, the difference between the placebo group and the 20-mg/d escitalopram group with respect to the percentage of subjects displaying 80% or greater reduction in the rating of the cardinal symptom of PMDD, that is, irritability, was considerable: 30% versus 80%. Adverse events were those normally reported in SSRI trials, such as nausea and reduced libido, and were not more common in patients given 20 mg/d of escitalopram than in patients given the lower dose. This study supports the usefulness of escitalopram for the treatment of PMDD and sheds further light on how different components of this syndrome are differently influenced by intermittent administration of an SSRI.