By systematically reviewing the literature, we have found that there is very little information about the frequency and clinical course of arteriovenous malformations (AVMs) of the brain in adults because the methods of most studies have been flawed, and AVMs tend to be treated once they are discovered. The incidence of AVMs is approximately 1 per 100 000 per year in unselected populations, and the point prevalence in adults is approximately 18 per 100 000. AVMs account for between 1 and 2% of all strokes, 3% of strokes in young adults, 9% of subarachnoid haemorrhages and, of all primary intracerebral haemorrhages, they are responsible for 4% overall, but for as much as one-third in young adults. AVMs are far less common causes of first presentations with unprovoked seizures (1%), and of people presenting with headaches in the absence of neurological signs (0.3%). At the time of detection, at least 15% of people affected by AVMs are asymptomatic, about one-fifth present with seizures and for approximately two-thirds of them the dominant mode of presentation is with intracranial haemorrhage. The limited high quality data available on prognosis suggest that long-term crude annual case fatality is 1-1.5%, the crude annual risk of first occurrence of haemorrhage from an unruptured AVM is approximately 2%, but the risk of recurrent haemorrhage may be as high as 18% in the first year, with uncertainty about the risk thereafter. For untreated AVMs, the annual risk of developing de novo seizures is 1%. There is a pressing need for large, prospective studies of the frequency and clinical course of AVMs in well-defined, stable populations, taking account of their prognostic heterogeneity.