A follow-up study of 153 consecutive patients who underwent complete excision of an angiographically visualized intracerebral arteriovenous malformation was conducted. The follow-up period ranged from 0.5 to 10.6 years, with a mean of 3.8 years. The presenting clinical event was hemorrhage in about one-half of the patients and seizure in about one-third. There was a marked tendency for postoperative neurological deficits to improve with time, so that whereas the immediate postoperative rate of serious morbidity was 24.2%, only 7.8% of the patients were found to have serious morbidity at follow-up. An additional 3 patients had died, one of an unrelated carcinoma, making the mortality related to arteriovenous malformation 1.3%. The classification of Spetzler and Martin (43) was used retrospectively; the percentages of Grade I (easiest) through Grade V (most difficult) lesions were 7.8%, 22.9%, 28.8%, 26.8%, and 13.8%, respectively. The early result was well correlated to grade, with good or excellent results in 100%, 94.3%, 88.6%, 61%, and 28.6% of the patients in Grades I through V, respectively. At follow-up, 98.7% of the patients with arteriovenous malformations of Grades I, II, and III were in good or excellent condition. The late morbidity and mortality rates for the patients in Grades IV and V were 12.2% and 38.4%, respectively. Of the patients who did not have seizures before surgery, 8.2% had only one or two seizures during the immediate postoperative period, and 7.1% had late seizures that were well controlled with medication in all. Of the patients who had seizures before surgery, over half were either cured or greatly improved with respect to the seizures.(ABSTRACT TRUNCATED AT 250 WORDS)