The postpartum period is a time when women are vulnerable to depressive disorders, which can be severe and have long-lasting adverse sequelae. In spite of multiple contacts with health care providers, women with postpartum depression often remain unrecognized and untreated. To evaluate the association between estradiol and postpartum depression, we measured serum estradiol concentration and performed an open-label study of physiologic 17beta-estradiol. Twenty-three women fulfilling ICD-10 criteria for major depression with postpartum onset were consecutively recruited from a psychiatric emergency unit. Serum estradiol concentrations were measured at baseline and weekly during sublingual 17beta-estradiol treatment for 8 weeks. The treatment effect was assessed using a clinician-rated depression symptom scale, the Montgomery-Asberg Depression Rating Scale (MADRS). At baseline, all patients were severely depressed (mean MADRS total score = 40.7; range, 35-45) and had a low serum estradiol concentration (mean = 79.8 pmol/L; range, 23-140 pmol/L); in 16/23 patients, the concentration was even lower than the threshold value for gonadal failure. During the first week of estradiol treatment, depressive symptoms diminished significantly, resulting in a mean MADRS score of 11.0 (Z = -4.20, p < .001), and serum estradiol concentrations approached those of the follicular phase (mean +/- SD = 342 +/- 141 pmol/L). At the end of the second week of treatment, the MADRS scores were compatible with clinical recovery in 19/23 patients. This preliminary study shows that depression symptoms may be rapidly reduced in patients with postpartum depression who have documented estradiol deficiency by treatment with 17beta-estradiol and suggests that estradiol can have significance in the pathophysiology of this condition and may be an option in the treatment of women vulnerable to postpartum depression.