Objective: Although there are studies in the literature focusing on the relationship between neurosteroids and psychiatric disorders, the studies on patients with obsessive-compulsive disorder (OCD) are limited in number. Nevertheless, allopregnanolone, a neurosteroid, in OCD patients has not been investigated in this limited number of studies. Allopregnanolone is considered to have a role in the pathogenesis of many psychiatric disorders. Accordingly, the present study aimed to investigate the relationship between the neurosteroid allopregnanolone and OCD. Methods: The study included 40 OCD patients diagnosed based on the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) diagnostic criteria and 40 healthy control subjects. The study participants completed the Yale-Brown Obsessive Compulsive Scale. Taking the diurnal rhythm of neurosteroids into consideration, blood samples for determination of plasma allopregnanolone levels were obtained from all patients and controls between 09:00 and 10:00 in the morning after 12 hours of fasting, tobacco abstinence, and 30 minutes of resting period. Plasma allopregnanolone level was analyzed using the enzyme-linked immunosorbent assay (ELISA). Results: No significant difference was determined between the patient and control groups regarding serum allopregnanolone levels. Evaluation of the allopregnanolone levels of the patients for each obsession type according to its presence and absence revealed no significant difference in any of them. Evaluation of the allopregnanolone levels of the patients for each compulsion type according to its presence and absence revealed that the allopregnanolone level of the patients with counting-organizing compulsive behavior was lower than that of those without. Discussion: Although there was no difference between OCD patients and controls regarding plasma allopregnanolone level, it is difficult to make a conclusion that allopregnanolone level does not play a role in the etiopathogenesis of disease. Conducting further studies, which would analyze allopregnanolone level in materials other than plasma such as cerebrospinal fluid using multiple analyses and separately evaluates females and males with higher number of patients, is of importance to enlighten at least some aspects of allopregnanolone-OCD relationship. Moreover, significantly lower plasma allopregnanolone level particularly in a single type of compulsion brings in mind again the suggestion that different etiopathogenesis might have a role in the symptom subtypes of OCD, which has begun to be discussed in the recent years. [Anadolu Psikiyatri Derg 2018; 19(5.000): 435-442]