Objective: Bipolar disorder is a severe mental disorder, and its prevalence is around 1% to 2%. Despite a vast literature around bipolar disorder, the reasons of its nonadherence and rehospitalization is still obscure. Several symptoms of bipolar disorder include changes in activity level, cognitive abilities, speech, and vegetative functions, such as sleep, sexual activity, as well as aggression, irritability, impulsive behaviors, and suicide. Owning to severity of symptoms, the first line of treatment is pharmacotherapy. After treating the acute phase of the disorder and controlling the symptoms, the patient is discharged from hospital with a relatively stable condition. Therefore, it is necessary and important for a patient to follow and adhere to the treatment. Methods:This research is a cross-sectional and prospective study. The study population includes all patients with bipolar disorder in Razi Psychiatric Hospital with a history of admission to psychiatric hospitals. A total of 73 patients with bipolar type I disorder with psychotic symptoms who had a history of admission to psychiatric hospitals with the same diagnosis, were recruited using a purposive, nonrandom sampling method. The study data were collected using medical records, semi-structured questionnaire (SCID), and a checklist. Results:The mean age of participants was 34.25 years, ranging from 20 to 51. It was found that only a few participants had a good treatment adherence (only 3.4%). The ANOVA and Chi-square tests showed demographic variables, such as gender, education, and age had effects on treatment nonadherence of patients with bipolar disorder. The results of linear regression analysis indicated that the t value for treatment adherence was significant (P<0.001). According to the regression model, patient's treatment adherence in the past predicts the nonadherence in the future. Furthermore, the results showed a correlation of 0.42 between behavioral and medication treatment adherence in participants (P<0.01).