Allopregnanolone is a neuroactive metabolite of progesterone and a barbiturate-like modulator of central gamma-aminobutyric acid receptors that modify a range of behaviors, including the stress response. The aim of this study was to determine the association of allopregnanolone levels with improvement of mood and behavioral symptoms following antidepressant treatment for severe premenstrual syndrome. A second exploratory aim was to determine whether allopregnanolone levels differed between antidepressant and placebo treatments. Serum samples from 46 women who were treated with sertraline, desipramine, or placebo in double-blind conditions were assayed. Improvement was assessed as the percent change from the pretreatment baseline in premenstrual symptoms, which were rated daily by the subjects. Twenty-seven samples were from improved subjects and 19 samples were from unimproved subjects following 2 to 3 months of double-blind treatment. Posttreatment allopregnanolone levels were significantly lower in the improved compared with the unimproved subjects. Improvement was also significantly associated with lower allopregnanolone levels for premenstrual depression and appetite changes. Improvement remained significantly associated with lower allopregnanolone levels after adjustment for treatment, cycle day of blood draw, age, and the interaction of treatment and cycle day. These preliminary results offer the first placebo-controlled evidence of association between allopregnanolone levels and premenstrual syndrome treatment response and suggest the importance of further study of the associations of allopregnanolone with premenstrual syndromes and the role of allopregnanolone in response to antidepressant medications.