To explore clinical benefit of performing two intrauterine inseminations (IUI) 24 h apart-a double IUI vs. a single IUI among lesbian and single women.Retrospective cohort study using electronic medical record review during a 17-year period (11/1999-3/2017). A total of 11,396 patients at a single academic-affiliated private practice were included in this study. All cycles with a single or double IUI were included. A sub-analysis of first cycles only (n = 10,413) was also performed. Canceled IVF cycles converted to IUI were excluded. T tests and Wilcoxon rank-sum tests were used for continuous data, and chi-square for categorical data. Multivariable logistic regression controlled for patient age, day 3 follicle-stimulating hormone (D3 FSH), body mass index (BMI), peak estradiol (E2), and post-wash total motile sperm counts to model the association between IUI number and ongoing pregnancy rate (OPR) according to sperm source (autologous vs. donor). Generalized estimating equations and mixed effect models accounted for multiple cycles from the same woman. Adjusted odds ratio (AOR) with 95% CI was determined. Sub-analyses of sexual orientation and partner status were performed to compare heterosexual couples with proven infertility to women with lesbian and single women.During the study period, 22,452 cycles met inclusion criteria (single IUI 1283 vs. double IUI 21,169). Mean patient age and BMI were similar between groups. For couples using autologous sperm, OPR was significantly higher with double IUI (12.0% vs. 14.1%; p = 0.0380). A similar increase was observed for donor sperm OPR among heterosexual couples (14.4% vs. 16.2%), though this did not reach statistical significance (p = 0.395). A sub-analysis restricted to donor sperm demonstrates a clinical benefit of second IUI in heterosexual couples, 8.5% vs. 17.6% OPR (AOR 2.94; CI 1.00-10.99; p = 0.0496). When lesbian and single patients were evaluated, there was no difference (17.2% vs. 15.2%; AOR 0.99; CI 0.59-1.70; p = 0.0958).Double IUI is associated with a significantly higher OPR for heterosexual couples using an autologous or donor sperm source. The benefit of a second IUI is less clear in patients with undocumented fertility status using donor sperm, such as single and lesbian women.