In a series of 1314 consecutive patients with cerebral aneurysms from a defined catchment area in eastern Finland (870,000 inhabitants), 561 patients (43%) had middle cerebral artery aneurysms (MCAAs). One or more associated aneurysms were common; 221 patients with MCAAs (39%) had multiple intracranial aneurysms (MIA). In other words, three-fourths (73%) of all patients with MIA had at least one MCAA. Multiple MCAAs, found in 111 patients (20%), were common in this Finnish population. One hundred of these patients had bilateral MCAAs, of whom 63 had mirror aneurysms, that is, aneurysms at the same site but on different sides. Thirty-five patients had "pure" mirror aneurysms, that is, they did not have any other aneurysms. Most MCAAs (81%) were located at the bifurcation. Three-fourths (72%) of the proximal MCAAs were associated with MIA. Giant aneurysms were significantly more common as single MCAAs than as any other single aneurysm. The frequency of intracerebral hematomas (42%) was by far higher in patients with MCAAs than in patients with ruptured aneurysms at other sites. Most of the intracerebral hematomas occurred in patients with bifurcation MCAAs that pointed laterally. Patients with MCAAs had surprisingly bad management outcomes despite good surgical results in patients with good Hunt and Hess grades. There were significantly more poor outcomes (Glasgow Outcome Scale score, 3-5) among patients with ruptured MCAAs than among those with any other anterior circulation aneurysms (32 and 25%, respectively). Also, the multiplicity of aneurysms increased the risk for poor outcome, which occurred in 39% of the patients who had MIA with one MCAA and 37% of those who had multiple MCAAs. Epilepsy, severe hemiparesis, and visual field deficits were the most common disabilities in long-term survivors, associated far more frequently with MCAAs than with aneurysm at other sites.