OBJECTIVE: In the addiction treatment, treatment drop-out rates are very high. Drop-out is a crucial problem in the addiction treatment; therefore, understanding the reasons underlying this problem will help clinicians in developing new treatment strategies for treatment maintainance. In this present study, we aimed to examine the reasons for drop-out behaviour in the outpatients who presented to an addiction counseling centre in Turkey. METHODS: This study was conducted in 554 outpatients with alcohol and/ or substance use disorder who presented to the Turkish Green Crescent Counseling Center (YEDAM) between January 2016 and July 2017. The patients were evaluated retrospectively. The sociodemographic characteristics, substance use-related and psychological characteristics were extracted by using the Addiction Profile Index (BAPI) and Addiction Profile Clinical Form (BAPI-K) included in the YEDAMSOFT software system. The patients who came to the second, fifth, and tenth sessions and the patients who dropped-out of the treatment were analyzed in order to determine the sociodemographic, substance use-related, and psychological characteristics that might caused patients to drop-out from the treatment. RESULTS: The drop-out rates was found higher in male patients compared to the females. Among the factors that caused drop-out behaviour were low education status, negative relationship with father, adverse effects of the substance use, severe desire for the substance use, low motivation level, being under probation, and higher numbers of psychiatric treatments in the past. We found that the factors predicted the drop-out may vary depending on the treatment session when drop-out from the treatment was actualized. The factors that predicted drop-out in the second sessions were severe desire for the substance use, being under probation order, history of psychiatric treaments in the past, and higher excitement seeking behaviour. The factors that predicted the drop-out after the fifth session were educational and marital statuses, adverse effects of the substance use, being under probation, and history of psychiatric treatments in the past. Age, educational status, and being under the probation were found as predictive factors for drop-out behaviour in the tenth session. The duration of the treatments were evaluated by Kaplan-Meier Survival Analysis. It was observed that the patients dropped-out after the 2.78th session from the first to the tenth session (CI = 2.58-2.98). It was found that the patients dropped-out after the 4.24th session from the second to the tenth session (CI = 3.98-4.51). In Cox Regression Analysis; psychiatric treatment history in the past and excitement seeking behaviour were the predictive factors for treatment drop-out with 95% confidence interval (p < 0.05). CONCLUSIONS: It was found that the factors that caused drop out from the treatment varied on session duration as it was hypothesized. Evaluating the factors that causes drop-out from the treatment will support clinicians to improve the addicti...