Research question: Endometrial polyps are a frequent finding during fertility treatment. Although up to 27% of small polyps (<10 mm) regress spontaneously, there is clinical benefit to removing a polyp detected before intrauterine insemination (IUI), regardless of size. However, the clinical outcome of IUI following a new suspicion of a polyp during follicle tracking is unknown. Design: This retrospective cohort study included all patients with a normal baseline uterine ultrasound and/or hysteroscopy result who started an IUI cycle between May 2009 and March 2017. In 139 of 6606 patients (2.1%), encompassing 340 out of 15,147 cycles (2.3% of cycles), a polyp was diagnosed during the follicular phase. The 6467 controls had ultrasound results with no suspicion of a polyp. Each patient was included only once in the analysis during a maximum of three consecutive cycles of IUI. Results: Female age was significantly higher in the polyp group than the controls (35.4 ± 4.8 versus 33.0 ± 5.0, P < 0.01). The unadjusted cumulative live birth rate (CLBR) after three IUI cycles in women with and without a polyp was 24.1% versus 33.0% (P = 0.03), indicating a deleterious effect of polyp(s). However, after multivariate Cox regression analysis for body mass index, female age, number of follicles and sperm concentration, the presence of a polyp appeared not to influence the CLBR (adjusted hazard ratio 0.742, 95% confidence interval 0.477–1.156, P = 0.19). Conclusions: These results may be reassuring, as ultrasound diagnosis of a polyp during the follicular phase of an IUI cycle does not seem to compromise clinical outcome when previous baseline examinations have been normal.