Peripheral blood CD3 , CD19 , CD4 , CD8 and CD45RO mononuclear cell subsets, T-cell proliferative responses to combinations of coimmobilized OKT3 antibody and an ECM protein (collagen I, collagen IV, ®bronectin or elastin), and T-cell adhesion to collagen IV, ®bronectin and elastin were studied in patients with aneurysmal subarachnoid haemorrhage. No signi®cant difference was found in the major lymphocyte subsets between subarachnoid haemorrhage patients receiving no dexamethasone for brain oedema treatment and healthy blood donors. Compared with the latter, both the dexamethasone-untreated and -treated subarachnoid haemorrhage patients showed decreased relative proliferative responses of circulating T cells to OKT3 combinations with collagen IV and ®bronectin, and an increased PHA-activated T-cell adhesion to elastin. CD45RO , CD4 and CD19 peripheral blood cell subsets, CD4 /CD8 cell ratio, PHA-activated T-cell adhesion to ®bronectin and collagen IV, and OKT3-triggered T-cell costimulatory responses to elastin, collagen IV and ®bronectin were signi®cantly higher in subarachnoid haemorrhage patients presenting with delayed cerebral vasospasm (DCV) than in their DCV-free counterparts. The DCV-related differences in circulating lymphocyte subsets showed no apparent relationship to the glucocorticoid treatment, whereas the differences in the other indices were con®ned to the dexamethasone-untreated subarachnoid haemorrhage patients. The above results suggest that the CD4 /CD8 ratio and T cell±ECM interactions play a role in the emergence of subarachnoid haemorrhage/DCV and may represent potential targets for subarachnoid haemorrhage therapy.