In 14 normal volunteers and 22 consecutive patients with subarachnoid hemorrhage (SAH) due to a ruptured cerebral aneurysm, ionized calcium (Ca²⁺) concentrations in whole blood were measured using an automatic calcium analyzer. The values were corrected against pH 7.4. The relationships between the Ca²⁺ concentration and the following aspects were studied: neurological grading by Hunt and Kosnik, grade of SAH on computed tomography scan by Fisher's definition, and the presence of vasospasm. Clinical vasospasm was recognized in eight cases. The averaged Ca²⁺ value of the control group was 1.23±0.02 mmol/l. In patients with a poor neurological grade or severe SAH, the Ca²⁺ level was apt to be lower than that of the control group. In patients with vasospasm, the values of Ca²⁺ were significantly decreased, especially between 8 and 14 days after SAH, compared with those patients without vasospasm and the control group (p< 0.05). These results indicate that measurement of Ca²⁺ concentration in whole blood may give a useful clue to treatment of vasospasm by calcium antagonist and that it may also provide a possible indicator as to the time of vasospasm in patients with severe SAH. However, it is very difficult to conclude whether decreased level of Ca²⁺ in patients with vasospasm is caused by the vasospasm itself.