Background The decision about whether to treat an unruptured brain arteriovenous malformation (AVM) depends on a comparison of the estimated lifetime risk of intracranial haemorrhage with the risks of interventional treatment. We aimed to test whether outcome differs between adults who had interventional AVM treatment and those who did not. Methods All adults in Scotland who were first diagnosed with an unruptured AVM during 1999-2003 (n=114) entered our prospective, population-based study. We compared the baseline characteristics and 3-year outcome of adults who received interventional treatment for their AVM (n=63) with those who did not (n=51). Findings At presentation, adults who were treated were younger (mean 40 vs 55 years of age, 95% CI for difference 9-20; p<0.0001), more likely to present with a seizure (odds ratio 2-4, 95% CI 1.1-5.0), and had fewer comorbidities (median 3 vs 4, p=0.03) than those who were not treated. Despite these baseline imbalances, treated and untreated groups did not differ in progression to Oxford Handicap Scale (OHS) scores of 2-6 (log-rank p=0.12) or 3-6 (log-rank p=0.98) in survival analyses. In a multivariable Cox proportional hazards analysis, the risk of poor outcome (OHS 2-6) was greater in patients who had interventional treatment than in those who did not (hazard ratio 2.5, 95% CI 1.1-6.0) and was greater in patients with a larger AVM nidus (hazard ratio 1.3, 95% CI 1.1-1.7). The treated and untreated groups did not differ in time to an OHS score of 2 or more that was sustained until the end of the third year of follow-up, or in the spectrum of dependence as measured by the OHS at 1, 2, and 3 years of follow-up. Interpretation Greater AVM size and interventional treatment were associated with worse short-term functional outcome for unruptured AVMs, but the longer-term effects of intervention are unclear.