Background: Hospitalization compared to outpatient care leads to better diagnosis. Stability of diagnosis varies among different psychiatric disorders and is associated with some demographic and mental health variables. Objective: The current study evaluated the stability of diagnosis in Baqiyatallah Hospital Psychiatric Ward. Methods: In this retrospective study, 908 inpatient records from the psychiatric ward of Baqiyatallah Hospital in the years 1997-2015 were randomly selected. Having primary and final diagnoses was the inclusion criterion. Demographic variables (age, sex, marital status, education, and employment) and mental health variables (primary and final psychiatric diagnoses, duration of hospitalization, psychiatric history, and medication history) were recorded. Ultimately, 429 cases were entered into the study. Results: The overall diagnostic stability rate was 57.6%. In mood, anxiety, psychotic, and personality disorders, the diagnostic stability rates were 84%, 63.8%, 46.3%, and 36.4%, respectively. For depressive and bipolar disorders, the stability of diagnosis rates were 85.5% and 86%, respectively. A significant relation between diagnosis axis, number of diagnoses, drug abuse and somatic disease history and diagnostic stability was seen (p<0.05). Conclusion: According to the present study, the maximum diagnostic stability rate was related to mood disorders with anxiety disorders ranking second. The minimum stability was related to personality disorders. Other studies have reported completely different results which may be due to different situations. Future studies in this field seem to be essential.