Aims: To study the effect of adenomyosis on in vitro fertilization/intracytoplasmic sperm injection (IVF/ICSI) outcomes in infertile patients. Methods: We performed a retrospective, database-searched cohort study based on 10,268 patients undergoing controlled ovarian hyperstimulation and IVF/ICSI between 2009 and 2011 in our unit. Adenomyosis was diagnosed by transvaginal ultrasound. A high-quality matched cohort study with strict inclusion criteria was conducted. We compared the basic characteristics and main IVF/ICSI outcomes between the two groups. Results: We identified 83 patients with adenomyosis, of whom we included 77, and strictly matched them to 77 patients without adenomyosis. Higher day 3 estrogen levels and a longer duration of gonadotropin stimulation days were found in women with adenomyosis compared to control subjects. Patients with adenomyosis had a nonsignificant trend toward a lower clinical pregnancy rate and a higher miscarriage rate (p = 0.103 and 0.09, respectively). The delivery rate was significantly lower in the adenomyosis group in comparison to the matched controls (p = 0.022). Conclusions: Within the limitations of a retrospective study (albeit with a remarkably large number of observations), our results suggest that transvaginal ultrasound-diagnosed adenomyosis could have a negative impact on the main IVF/ICSI outcomes. Improving the diagnostic validity and scoring of disease severity in patients with adenomyosis is suggested.