It has been reported that the reduction of cerebral blood flow (CBF) is more pronounced with thalamic hemorrhages than with putaminal hemorrhages, and the clinical outcome is worse with the former. However, the mechanism underlying these differences is not clear. We compared neurologic status, hematoma volumes, outcome scores. and early (< 1 month) and late (2-12 month) CBF values between 15 patients with thalamic hemorrhages and 28 patients with putaminal hemorrhages. We also correlated thalamic versus hemispheric CBF on each side and ipsilateral versus contralateral thalamic and hemispheric CBF. Finally, we evaluated the response to acetazolamide during the late stage. Thalamic hemorrhages were associated with a more pronounced reduction in CBF bilaterally, even though their hematoma volumes were much smaller. Contralateral to the hemorrhage, the discrepancy in CBF values between the two groups became greater in the late stage because CBF started to recover in putaminal hemorrhages but persistently deteriorated in thalamic hemorrhages. In the group with thalamic hemorrhages, the correlation between thalamic and hemispheric CBF ipsilateral to the hemorrhage and between thalamic CBF on both sides was disrupted in the early stage and restored in the late stage, whereas the correlation between the hemispheric CBF values was consistently preserved. Acetazolamide invariably augumented CBF during the late stage. The clinical outcome was worse in the thalamic group, but CBF values correlated negatively with outcome in both groups. We conclude that the reduction of CBF in the late stage may be secondary to metabolic depression due to transneural depression ('diaschisis'). The metabolic depression in thalamic hemorrhages is more extensive and persistent than in putaminal hemorrhages, which probably accounts for both the more pronounced CBF reduction and the worse outcome.