The association of uterine leiomyoma with increased risk of breast cancer is controversial. Therefore, we used the National Health Insurance Research Database of Taiwan to examine breast cancer incidence and mortality among Asian patients with and without uterine leiomyoma. We compared breast cancer incidence and mortality between 22,001 newly diagnosed uterine leiomyoma patients and 85,356 individuals without uterine leiomyoma matched by age and date of diagnosis. Adjusted hazard ratios for breast cancer were estimated using the Cox model. The incidence of breast cancer was 35% higher in the uterine leiomyoma group than the leiomyoma-free group (1.65 vs. 1.22 per 1,000 individuals, p < 0.001), with an adjusted hazard ratio of 1.31 (95% confidence interval = 1.13−1.52). Interestingly, overall mortality was lower (4.12%) in the uterine leiomyoma group (mean followed time, 3.59 ± 2.70 years) than the leiomyoma-free group (8.78%; mean followed time, 3.54 ± 2.67 years) at the endpoint of the study (p <0.05). These findings indicate the incidence of breast cancer is higher in patients with uterine leiomyoma than in those without it, but overall mortality from breast cancer was lower in the patients with uterine leiomyoma.