The Swedish psychiatric inpatient register starts in 1971 and covers all inpatient treatments since 1973. For each hospitalization, the unique national registration number, date of admission and discharge, and diagnosis are registered. No private inpatient facilities exist in Sweden, so the psychiatric inpatient register is therefore population based. The diagnoses were recorded according to the International Classification of Diseases, Eighth Revision (ICD-8), from 1971 to 1986, and according to the International Classification of Diseases, Ninth Revision (ICD-9), from 1987 to 1995. All diagnoses in the register are made by a consultant in psychiatry at the time of the patient's discharge from the hospital. Several diagnoses can be recorded at each admission, but only the first (main) diagnosis was used in this study. To exclude readmitted cases, admissions in the inpatient register were observed until the end of 1972, and all individuals with a relevant diagnosis before 1973 were excluded. An upper age limit at the first diagnosis was set at 70 years. All patients with a first diagnosis of bipolar disorder or unipolar disorder from January 1, 1973, to December 31, 1995, were identified in the psychiatric inpatient register. Bipolar disorder was assessed from ICD-8 as manic-depressive psychosis manic type (296.10), manic-depressive psychosis circular type (296.30), and reactive excitative psychosis (298.10), and from ICD-9 as unipolar affective psychosis manic form (296 A), bipolar affective psychosis manic phase (296 C), bipolar affective psychosis mixed form (296 E), and reactive excitative psychosis (298 B). Unipolar disorder was defined from ICD-8 as melancholic involution psychosis (296.00), manic-depressive psychosis depressive type (296.20), manic-depressive psychosis alia definita (296.88), manic-depressive psychosis not otherwise specified (296.99), and reactive depressive psychosis (298.00), and from ICD-9 as unipolar affective psychosis melancholic form (296 B), bipolar affective psychosis melancholic phase (296 D), unspecified affective psychosis (296 X), and reactive depressive psychosis (298 A).