Expansion of intracerebral hematoma usually occurs in the chronic phase because of repeated bleeding from pre-existing vascular anomalies or exudation of blood from capillaries of the capsule. In contrast, spontaneous intracerebral hematoma expanding during the acute phase of hemorrhage without rebleeding is seldom seen. Three such cases are reported, along with magnetic resonance (MR) and computerized tomography (CT) follow-up studies. The follow-up MR images and CT scans demonstrated no evidence of rebleeding, but revealed gradual expansion of a fluid component of the hematoma, beginning in the acute phase. Volume alterations posthemorrhage are carefully documented. There was a characteristic phenomenon of layering, with the red blood cell component of the clot settling by gravity and the serum separating as well as seen in a test tube, depending on whether a preservative was used. Examination of blood samples indicated a possible correlation between expansion of the hematoma and the activities of both the fibrinolytic system and coagulation factors.