Objective: Diagnostic stability is the degree to which a diagnosis remains unchanged during time. Our main objective was to evaluate the diagnostic stability of bipolar disorder (BD) in psychiatric outpatient consultations and determine the socio-demographic variables influencing its stability., Methods: The Cumulative Register of Cases of the Community of Madrid provided data on all outpatient visits conducted at Madrid's Community Mental Healthcare Centers between 1980-2009. Diagnoses were made according to ICD-9 / ICD-10 criteria. Two indices were measured: temporal consistency (maintenance of the diagnosis over time) and diagnostic constancy (presence of BD diagnosis in at least 75% of visits). κ coefficient measured the agreement between diagnoses in the first and last evaluations (prospective and retrospective consistency)., Results: 14,557 patients were diagnosed with BD for at least 1 evaluation and had at least 10 visits and 1 year of follow-up. At first evaluation, 3,988 patients were diagnosed with BD (prospective consistency 50.8%), and at last evaluation 5,396 patients were diagnosed with BD (retrospective consistency 37.5%). A total of 2,026 patients were diagnosed with BD at their first and last evaluations (prospective consistency 18.3%)., Conclusions: This longitudinal study conducted in community mental health centers reflects common diagnostic practices in outpatient settings over a 30-year period (130,000 patient-years). Delay of > 10 years was found to achieve diagnostic stability. Frequent diagnostic shifts were found in relation to BD, the most common being with other affective disorders. Anxiety was also a common misdiagnosis. Greater stability was associated with having been diagnosed after hospitalization, having an age at onset > 25 years, and having an age at diagnosis < 24 years., (© Copyright 2021 Physicians Postgraduate Press, Inc.)