Serial changes in platelet and white blood cell (WBC) counts and other blood parameters were analyzed in 103 patients with aneurysmal subarachnoid hemorrhage (SAH). The WBC counts during days 3-5, 6-8, 9-11, and 12-14 after the onset of SAH were significantly higher in patients with than in patients without symptomatic vasospasm. Platelet counts during days 0-2, 3-5, 6-8, 9-11, 12-14, 15-17, 18-21, and 22-28 after SAH were significantly higher in patients with than in patients without symptomatic vasospasm. Monitoring of platelet and WBC counts may provide an indicator of the occurrence of symptomatic vasospasm.