Three cases of histologically proven cryptic arteriovenous (AV) malformation of the basal ganglia are reported. Seventeen cases of patients showing putaminal hemorrhage in the CT scan underwent craniotomy for evacuation of a hematoma between January 1986 and December 1988. The preoperative diagnosis was hypertensive putaminal hemorrhage. In all three cases a network of abnormal vessels around the internal capsule was detected at surgery. Case 1: A 62-year-old male was admitted to our hospital with consciousness disturbance (JCS 20), total aphasia and right hemiparesis. The CT scan showed left putaminal hemorrhage. The volume of the hematoma was 45 ml by CT criteria. Left carotid angiography did not demonstrate any abnormal findings. Case 2: 69-year-old male was admitted with consciousness disturbance (JCS 10) and right hemiparesis. Left putaminal hemorrhage (volume 25 ml) was recognized on the CT scan. Two hours after admission, the consciousness level suddenly deteriorated (JCS 100). At that moment, the volume of the putaminal hematoma had increased to 100 ml. AV malformation was not detected by left carotid angiography. Case 3: A 50-year-old male was admitted with consciousness disturbance (JCS 20), total aphasia and right hemiparesis. The CT scan showed left putaminal hemorrhage of 73 ml. Pathohistological examination proved AV malformation in each case. The three cases described here suggest the following; 1) The incidence of the basal ganglia cryptic AV malformation has been considered low, but meticulous examination shows that among the cases diagnosed as hypertensive hemorrhage, AV malformation is not infrequently the hidden cause.(ABSTRACT TRUNCATED AT 250 WORDS)