This study was designed to evaluate the clinical value of consecutive measurements of the levels of the anticoagulation factors protein C. protein S and antithrombin III in the plasma of patients with subarachnoid haemorrhage (SAH). Sequential samples of plasma were obtained from 34 patients at days 0 to 3, 5, 8, 14, and 20 days after SAH. Their levels of protein C, protein S and antithrombin III were measured by specific immuno-assays, and the symptomatic vasospasm and clinical states of the patients were monitored. Based on the retrospective comparisons between the measured levels of the three factors and the levels in pooled plasma from normal volunteers, the time course of changes in the levels allowed the classification of the patients into five groups as follows: A, the levels of the three factors decreased suddenly at 5 days after SAH, then increased again (n = 6); B, the three factors increased continuously (n = 13); C, the three factors remained below the normal range (n = 3); D, the three factors remained within the normal range (n = 6); and E, the three factors decreased continuously (n = 6). In group A patients, temporary deficiencies in the three factors occurred that coincided with the occurrence of delayed neurological deficit (DND). Twelve patients (92%) in group B had a favourable outcome (good and moderate disability) without DND. The patients in group C had low levels of anticoagulation factors and mild DND followed by a favourable outcome. Five patients (83%) in group D had a favourable outcome. In group E, five patients (83%) had a poor outcome (severe disability and death) due to DND. When clinical outcomes were compared by patient group, the patients in group E had significantly more unfavourable outcomes than did the other groups (P < 0.01). Although the mechanism responsible for the decrease in anticoagulation factor levels remained unclear, the results suggested that the decrease in anticoagulation factors may indicate hypercoagulability and cerebral ischaemia after SAH.