OBJECTIVES: To determine the prevalence of affective disorders (section F3 in ICD-10) in South-West region of Sweden. METHODS: This was a retrospective longitudinal descriptive database study of the utilization of healthcare of patients from the South-West region of Sweden (1.5 million inhabitants). All patients who were diagnosed with affective disorders between 2001 and 2007 in the South-West region of Sweden were included in the study. The patients were diagnosed in primary-, in- and out-patient care. RESULTS: A total of 138,194 patients were diagnosed with at least one type of affective disorder. This corresponded to a prevalence of 9% in the population of South-West region in Sweden. 89 percent (123,524 patients) of the total sample were diagnosed with major depressive disorder (F32) followed by recurrent depressive disorder (F33) at 14% (19,944 patients). Bipolar affective disorder (F31) was diagnosed in 5% of the patients (6932) and unspecified mood (affective) disorder (F39) was diagnosed in 4% of the patients (5,431). The prevalence of the different diagnoses were 8 percent for depressive episode, 1 percent for recurrent depressive disorder, 0.5 percent for bipolar affective disorder, 0.4% for unspecified mood (affective) disorder. CONCLUSIONS: Depressive episode was by far the most common of the affective disorders diagnosed, with a prevalence of 8 percent. A question remains whether the prevalence for bipolar is underestimated as this study found it to be 0.5%. The expected prevalence for bipolar disorder is 1 percent of the population. One plausible explanation could be that bipolar patients are classified with other codes than F31 in the South-West region of Sweden.