To determine which dose of transdermal E2 could be administered to breastfeeding women without significant transfer into breast milk. Single center, prospective study. University hospital research unit. Twenty one healthy breastfeeding women (aged 20- 38 years) who had delivered 20 weeks earlier. Treatment with either 50, 75, or 100 μ g/24 hours of transdermal E2 or placebo for 2 weeks beginning at 20 weeks postpartum. Breast milk and blood samples were collected before the treatment and 2 weeks into the treatment. Serum and milk E2 were measured by RIA after diethyl ether ethyl acetate extraction. Serum inhibin B concentrations were measured by ELISA and serum FSH and LH concentrations were measured by fluoroimmunoassay. None of the breast milk samples contained any measurable concentrations of E2 after E2 treatment. Serum E2 concentrations were elevated in a dose dependent manner. Both serum FSH and LH concentrations were decreased in all treatment groups with a more pronounced suppression in the 75 and 100 μ g/24- hour groups. Serum inhibin B concentrations were not significantly suppressed during the study. Three of six women using the 100 μ g/24- hour E2 dose discontinued the study due to side effects. A dose as high as 100 μ g/24 hours of E2 can be administered transdermally without traces appearing in the breast milk. Whether the symptoms associated with the hypoestrogenic state of lactation can be alleviated by transdermal E2 treatment (50 or 75 μ g/24 hours) can now be studied in the early postpartum phase (6 weeks onward). To determine which dose of transdermal E2 could be breastfeeding women without significant transfer into breast milk. Single center, prospective study. University hospital research unit. Twenty one healthy breastfeeding women (aged 20-38 years) who had delivered 20 weeks earlier. Serum and milk E2 were treated with either 50, 75, or 100 μg / 24 hours of transdermal E2 or placebo for 2 weeks beginning at 20 weeks postpartum. Breast milk and blood samples were collected before the treatment and 2 weeks into the treatment. measured by RIA after diethyl ether ethyl acetate extraction. Serum inhibin B concentrations were measured by ELISA and serum FSH and LH concentrations were measured by fluoroimmunoassay. None of the breast milk samples contained any measurable concentrations of E2 after E2 treatment. Serum E2 concentrations were elevated Both serum FSH and LH concentrations were decreased in all treatment groups with a more pronounce d suppression in the 75 and 100 μg / 24-hour groups. Serum inhibin B concentrations were not significantly suppressed during the study. Three of six women using the 100 μg / 24-hour E2 dose discontinued the study due to side effects. Whether the symptoms associated with the hypoestrogenic state of lactation can be alleviated by transdermal E2 treatment (50 or 75 μg / 24 can now be studied in the early postpartum phase (6 weeks onward).