Vaginal atrophy symptoms such as dryness, irritation, and itching, are common after menopause. Vaginal estrogen therapy is the most effective treatment but not appropriate for all women. Women with estrogen-responsive breast cancer treated with aromatase inhibitor (AI) treatment, suppressing estrogen levels, often suffer from more pronounced vaginal atrophy symptoms. However, vaginal estrogen treatment is not recommended, leaving them without effective treatment options. The aim of this thesis was to study the effect of long-term anti-estrogen therapy on circulating estrogen levels and biochemical factors in vaginal mucosa in relation to morphological changes and clinical signs of vaginal atrophy. Circulating estrogen levels were analyzed by use of mass spectrometry and radioimmunoassay. Immunohistochemistry was used to study vaginal proliferation and steroid hormone receptors in vaginal mucosa. Vaginal gene expression was studied by use of microarray technology and bioinformatic tools, and validated by use of quantitative real-time PCR and immunohistochemistry. An estrogenic regulation of aquaporins and a possible role in vaginal dryness was investigated in vaginal mucosa and in Vk2E6E7 cells. Aromatase inhibitor-treated women had higher than expected estradiol and estrone levels but still significantly lower than other postmenopausal women. Aromatase was detected in vaginal tissue, the slightly stronger staining in vaginal mucosa from AI-treated women, suggest a local inhibition of vaginal aromatase in addition to the systemic suppression. Vaginal mucosa from AI-treated women had weak progesterone receptor, and strong androgen receptor staining intensity. Low estrogen levels lead to low expression of genes involved in cell adhesion, proliferation, and differentiation as well as weak aquaporin 3 protein immunostaining. The higher than expected estrogen levels in AI-treated women suggest that estrogen levels might previously have been underestimated. Systemic estrogen suppression by treatment with AIs, and possibly also by local inhibition of vaginal aromatase, results in reduced cell adhesion, proliferation, differentiation, and weak aquaporin 3 protein staining. Low proliferation and poor differentiation leads to fewer and less differentiated superficial cells affecting epithelial function and possibly also causing vaginal symptoms. Aquaporin 3 with a possible role in vaginal dryness, cell proliferation, and differentiation should be further explored for the development of non-hormonal treatment options for vaginal symptoms.