Abstract Study question Does sperm reactive oxygen species values affect the pregnancy rate after Intrauterine Insemination (IUI)? Summary answer Sperm reactive oxygen species values affect pregnancy rate and can be used as a predictive marker for IUI outcome. What is known already Oxidative stress resulting from excessive production of reactive oxygen species (ROS) can profoundly affect the sperm and subsequent functional sperm integrity. Diagnostics to measure semen oxidative stress have been added to the 6th edition of the WHO laboratory manual for examining and processing of human semen, under advanced examinations. Although a semen analysis has classically been used as the gold standard for determining a man’s fertility, basic sperm analysis alone cannot accurately predict infertility since between 6 and 27 % of men with normal semen parameters are infertile. Determination of oxidative stress in diagnosing infertility may contribute to better outcomes. Study design, size, duration This study was based on semen analyses of 50 male patients whose samples were used for IUI treatment between November 2022 and January 2023. The oxidation-reduction potential was measured using novel galvanostat-based technology, the MiOXSYS System. Briefly, 30 μL of liquefied semen at room temperature were applied to the MiOXSYS sensor. Both absolute ORP (mV) and normalized sORP values ( mV/10 6 sperm/ml) based on sperm concentration were calculated. Participants/materials, setting, methods Sperm analysis were performed according to the WHO laboratory manual sperm criteria (Sixth edition 2021). All participants followed a period of 3–5 days of sexual abstinence and had a diagnosis of unexplained infertility (normozoospermia). Ovarian stimulation for IUI cycle was initiated on the 5th day of menstruation by using recombinant gonadotropins. Clinical pregnancy was determined based on the Beta-hCG values. For statistical comparison between group values the Mann-Whitney U test was used. Main results and the role of chance According to the guidelines of the World Health Organization, all participants included in the study had normal BMI (21.85±3.80 kg/m2). The average age of the female patients included in the study was 33.9 ± 3.1 years while the average age of the male patients was 37.8 ± 4.8 years. Out of the 50 patients, 32% (N = 16) got pregnant. For the study propose sperm ORP and sORP values were compared between patients based on the pregnancy outcome. Statistical analysis showed a significant difference in ORP values. Patients with a positive pregnancy outcome after IUI had a lower ORP value compared to patients where IUI was not successful [25.65 [6.7-52.6] mV vs. 40.2 [6.3-109.8] mV, respectively, p = 0.02]. Additionally, normalized values according to the sperm concentration (sORP) were also significantly different between those groups [ 0.37(0.17-1.44) mV/10 6 sperm/ml vs. 0.88(0.13-20.81) mV/10 6 sperm/ml, respectively p = 0.006]. Due to the above-mentioned significant difference logistic regression analysis were performed, and the obtained results confirm that sperm ORP (p = 0.02) as well sORP (p = 0.016) significantly affects the pregnancy rate after the IUI procedure. Limitations, reasons for caution The limitation of the presented study was a relatively small patient cohort. Additionally, pregnancy was determined based on the beta hCG values, therefore future research should link ORP values and live birth rate as well. Wider implications of the findings In the present study suggests ORP as an additional parameter with diagnostic value to identify the best therapeutic approach concerning IUI in couples that show minimal or no impairments of the WHO semen parameter. Trial registration number not applicable