To determine whether the presence of focal adenomyosis of the outer myometrium (FAOM) at preoperative magnetic resonance imaging is associated with the severity of deep infiltrating endometriosis. Observational cross-sectional study involving 255 symptomatic deep infiltrating endometriosis patients. Comparisons were performed according to the presence of FAOM. University hospital. Women with a preoperative magnetic resonance imaging and complete surgical exeresis of endometriotic lesions with histologically documented deep infiltrating endometriosis. Surgical management for deep infiltrating endometriosis. The presence of multiple deep infiltrating endometriosis lesions, the mean number and location of deep infiltrating endometriosis lesions, and the mean total revised American Society for Reproductive Medicine scores. The prevalence of FAOM at preoperative magnetic resonance imaging in the 255 patients with deep infiltrating endometriosis was 56.5%. The mean number of deep infiltrating endometriosis lesions was significantly higher in the FAOM(+) group than in the FAOM(-) group: 3.5 ± 2.1 vs. 2.2 ± 1.5. The mean total revised American Society for Reproductive Medicine score was higher in case of FOAM coexisting with deep infiltrating endometriosis. After adjusting for confounding factors, the presence of FAOM was significantly associated with multiple deep lesions. FAOM was significantly associated with greater deep infiltrating endometriosis severity. This needs to be integrated into the management strategy. Furthermore, a pathogenic link between deep infiltrating endometriosis and FAOM cannot be excluded.