OBJECTIVE: Our purpose was to evaluate the effect of estrogen replacement therapy on sleep complaints by postmenopausal women and to assess the predictive factors involved. STUDY DESIGN: Sixty-three postmenopausal women entered a 7-month prospective, randomized, double-blind, crossover study consisting of two 3-month treatments with estrogen and placebo with a 1-month washout period between. Eight Visual Analogic Scale statements about different sleep complaints, the Basic Nordic Sleep Questionnaire, scoring of climacteric symptoms, The Beck Depression Inventory, and serum estradiol and follicle-stimulating hormone level controls were the main outcome measures. RESULTS: Estrogen replacement therapy improved sleep quality, facilitated falling asleep, and decreased nocturnal restlessness and awakenings (p < 0.001). The subjects were less tired in the mornings and in the daytime (p < 0.001) when taking estrogen replacement therapy. Estrogen-induced sleep improvement was associated with alleviation of vasomotor symptoms (r range 0.27 to 0.55), alleviation of somatic symptoms (palpitations and muscular pain, r range 0.26 to 0.36), and alleviation of mood symptoms (r range 0.28 to 0.37) on estrogen replacement therapy. The severity of initial insomnia predicted only one estrogen-induced sleep improvement effect: the more the subjects experienced insomnia, the better the estrogen replacement therapy facilitated falling asleep (r = 0.26, p = 0.040). Estrogen-induced sleep improvement was also reported by the 15 climacterically asymptomatic subjects. In these subjects initial insomnia scores strongly predicted estrogen-induced sleep improvement (r range 0.50 to 0.75). CONCLUSIONS: Estrogen replacement therapy significantly diminished sleep complaints among postmenopausal women. Alleviation of climacteric symptoms was the most important predictive factor for the beneficial effect of estrogen replacement therapy on sleep complaints. The use of estrogen replacement therapy in women without self-reported climacteric symptoms could also be considered because women do not always recognize their climacteric symptoms or they ignore them. (Am J Obstet Gynecol 1998;178:1002-9.)