OBJECTIVE: To assess whether a pH-balanced vaginal gel containing lactic acid is more effective than a placebo (lactate-free gel) in improving dyspareunia and sexual function among breast cancer survivors who were premenopausal at diagnosis and had dyspareunia after adjuvant chemotherapy. METHODS: In a single-center, double-blind, randomized trial, a pH-balanced gel or placebo was administered three times per week at bedtime as well as during sexual intercourse for 8 weeks. The primary outcome was the improvement of dyspareunia measured by pain score of the Female Sexual Function Index after the treatment. Secondary outcomes included the total and individual domains of Female Sexual Function Index score, sexual dysfunction (a total Female Sexual Function Index score less than 25.0), vaginal pH, vaginal maturation index, and adverse events related to the intervention. A sample size of 47 per group was planned to achieve 80% power to detect a 19% difference in the primary outcome. RESULTS: From October 2009 and March 2013, 167 women were screened and 136 were randomized: 69 to a pH-balanced gel and 67 to placebo. Baseline characteristics were similar in both groups. Although there was no difference between the two groups, both experienced a significant improvement of dyspareunia. The increase in median pain score from baseline was 1.2 in both groups (median [interquartile range] from 2.8 [2.0-4.0] to 4.0 [2.8-4.8] in the pH-balanced group and from 3.2 [2.0-4.0] to 4.4 [3.2-4.8] in the placebo group; all P,.01). Overall Female Sexual Function Index score and the frequency of sexual dysfunction also did not differ between the two groups although there was a significant improvement. On the other hand, vaginal pH and vaginal maturation index were slightly but significantly improved only in the pH-balanced group. There were no severe adverse events in either group. CONCLUSION: The pH-balanced vaginal gel is not superior to the placebo in improving dyspareunia and overall sexual function.