Background: Recent reports have suggested that bipolar disorder beginning in late life is strongly associated with organic brain disease whereas early-onset cases are more likely to be associated with a family history of mood disorder. It is not yet clear whether late-onset bipolar disorder is therefore a “phenocopy” of the classic early-onset disorder, sharing symptoms but having a different etiology, or whether people with early- and late-onset bipolar disorder have a common underlying vulnerability that interacts with age-specific triggering factors. Aim: The present study examines the administrative records of patients treated for bipolar disorder, to establish whether differences between early- and late-onset cases might be consistent with their having distinct etiological processes. Methods: We used a file containing administrative data for all patients with a diagnosis of bipolar disorder who were in contact with the health services of Western Australia between 1980 and 1998. For each contact with psychiatric services, the file provided the patient's age, gender, marital status, educational achievement, employment, ethnic origin, postcode of residence, primary and secondary diagnoses, and the duration of the (administrative) episode. Subjects were designated “late-onset” when their first contact with psychiatric services occurred at or after 65 years of age. Results: Between 1980 and 1998 there were 33,004 service contacts involving 6,182 individuals whose primary or secondary clinical diagnosis was bipolar disorder. This indicates that the prevalence of bipolar disorder in Western Australia is approximately 0.4%. Most patients had an onset of illness between 15 and 45 years of age, but 492 patients (8%) were aged 65 years or over at the time of first contact with mental health services. We observed that the relative frequency of late-onset bipolar disorder increased between 1980 and 1998 (1% to 11%). There was an excess of women in our cohort (3:2), but no difference in the age of onset between males and females. Early onset was associated with a subsequently lower socioeconomic status, aboriginal ethnicity, and a higher frequency of mixed affective episodes, other mood disorders, schizophrenia, and schizoaffective disorder. Patients with late-onset bipolar disorder were more likely to have a diagnosis of organic mental disorder recorded (2.8% vs. 1.2%). There was no evidence of a bimodal pattern of age-specific incidence. Conclusion: The observed differences between early- and late-onset bipolar disorders are small and most likely attributable to differences in the duration of illness. Only a small proportion of patients with bipolar disorder were ever diagnosed with an organic mental disorder, which suggests that the reported association between late onset of illness and organic factors may be of limited clinical relevance.