Abstract This study investigated the characteristics and medical practice of hospitalized patients with ovarian teratomas complicated by anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis in Japan. This cohort study was conducted using “Diagnosis Procedure Combination” (DPC) codes, a patient classification method for acute phase inpatients, over a 6-year period (2014–2020). Hospitalizations for ovarian teratomas complicated with anti-NMDAR encephalitis were compared with those for ovarian teratomas uncomplicated with anti-NMDAR encephalitis. Additionally, the tendencies of patients with ovarian teratomas complicated with anti-NMDAR encephalitis and the clinical management were considered. Ovarian teratoma patients with encephalitis had a younger median age than those without encephalitis. The group with ovarian teratoma complicated with anti-NMDAR encephalitis was more likely to be transported by ambulance and had more transfers from other hospitals than the uncomplicated group. Among the target hospitalized patients, the 15 − 19 age group had the highest incidence rates. The median age was lower for steroid pulse therapy, intravenous immunoglobulin therapy, and plasma exchange therapy, in that order. Additionally, younger patients underwent more invasive treatments. Anti-NMDAR encephalitis associated with ovarian tumors often occurs in young people in their late teens and 20s, and younger patients are more likely to be severely ill.
