One hundred and five patients with hypertensive cerebral hemorrhage or cerebral infarction were reviewed with regard to the incidence of the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). SIADH occurred in 8 (7.6%) out of 105 patients. The mean interval between the onset of cerebrovascular accidents (CVA) and the development of SIADH was 13.9 days (ranging from 7 to 23 days). All the cases with SIADH showed moderate to severe disturbance of consciousness levels. In 2 of these 8 cases, preexisting impaired consciousness was aggravated with severe hyponatremia. The incidence of SIADH varied significantly according to the type of CVA, being lower in cases with cerebral infarction [one out of 51 (1.9%)], than in cases with cerebral hemorrhage [7 out of 54 (12.9%)]. The incidence of SIADH in patients with thalamic hemorrhage (26.7%) was much higher than in patients with putaminal hemorrhage (7.7%). Plasma antidiuretic hormone (ADH) levels measured in 2 cases with SIADH were 8.1 and 6.5 pg/ml, inappropriately elevated for the corresponding serum osmolarities. Even though the mean values of serum sodium and osmolarity were not significantly different from the controls, the mean (±SD) plasma level of ADH in cases with cerebral hemorrhage measured within 14 days after the onset was 7.3±1.6 pg/ml, significantly higher than the controls (p<0.001). Hematological and urinary studies showed that the excessive release of ADH recognized in cases with cerebral hemorrhage was a non-physiological (inappropriate) release of ADH, caused by non-osmotic factors, except blood volume depletion. From the present study, it is suggested that patients with impaired consciousness, especially in an acute stage following cerebral hemorrhage, are unable to suppress ADH release properly to excrete the water load normally, and are susceptible to SIADH even if they are under normoosmotic conditions.