OBJECTIVE: Atrophic Papanicolaou (Pap) smears from postmenopausal women may be unsatisfactory for assessment or result in a false-positive diagnosis of a cytological abnormality. We investigated the effect of vaginal estrogen treatment before the Pap test on the odds of an atrophic smear. METHODS: An open-label randomized controlled trial was conducted to compare the proportion of atrophic Pap smears from postmenopausal women assigned to either (1) a regimen of one 25-microg vaginal estradiol tablet inserted nightly for five nights before their Pap test, (2) a single 25-microg vaginal estradiol tablet before the test, or (3) a control group with no previous estrogen administration. All smears were reread and classified as atrophic or nonatrophic at the conclusion of the study by a single cytopathologist who was blinded to the study arms. RESULTS: One hundred fifty-four (94%) of the 164 postmenopausal women who consented to the study were included in the final analysis. Fifty-one women had received the five-night course of tablets, 50 had received one tablet, and 53 were assigned to the group with no previous estrogen use. The odds of an atrophic smear were significantly lower in women who used the five-night estrogen regimen than in women who did not use estrogen. The estimated odds ratio of an atrophic smear in the five-night regimen was 0.01 (95% CI, 0.03-0.26) compared with the no-estrogen control group. Moreover, using one tablet of estrogen had no significant effect on the likelihood of an atrophic smear compared with using none. The odds ratio of an atrophic smear in the single estrogen tablet group was 1.05 (95% CI, 0.48-2.29) compared with the no-estrogen group. CONCLUSIONS: The odds of an atrophic smear are significantly reduced for postmenopausal women who use a five-night regimen of vaginal estrogen before their Pap test.