The time in therapeutic range (TTR) of international normalized ratio (INR) is essential for the safety and efficacy of warfarin treatment. In this study, we aimed to determine TTR and the factors that affect TTR in patients using warfarin.Patients taking warfarin for valvular and nonvalvular atrial fibrillation (AF) or prosthetic heart valves who were admitted to our cardiology outpatient clinic were enrolled. TTR was calculated using the linear interpolation method. The patients were analyzed according to warfarin indications and TTR efficiency (TTR ≥60%). Weekly warfarin dose, the duration of warfarin use, the frequency of INR visits per year, and the awareness of patients regarding target INR were noted.The TTR of 248 patients (aged 57.21±12.45 years, 33.1% male) was 55.92±27.84%, and 48.0% patients exhibited efficient TTR. Clinical and demographic characteristics (age, sex, socioeconomic status, and comorbidities) exerted no effect on TTR and TTR efficiency. The frequency of INR visits per year was 10.02±3.80. TTR was related to the frequency of annual INR visits (r=0.131, p=0.039). Only one-third (30.2%) of patients were aware of their target INR. The literacy of the patients and duration of warfarin use exerted a positive effect on awareness (p=0.011 and p=0.024, respectively).The findings of our study demonstrated that TTR and TTR efficiency were low and not associated with the characteristics of patients or indications. Unfortunately, in patients with valvular AF and prosthetic valves, warfarin is the sole drug that can be used. Thus, awareness and knowledge regarding target INR are essential to overcome poor anticoagulation monitoring with frequent INR visits.