Plasma viscosity, fibrinogen, haematocrit and beta-thromboglobulin were assessed on venous blood samples taken within 24 hours of admission from 20 consecutive male hypertensive patients with unstable angina and 20 male hypertensive patients with stable angina, matched for clinical variables. Besides, all patients underwent automated indirect blood pressure monitoring for 24 hours, starting just after hospitalization. Despite similar average 24-hour, day-time and night-time systolic and diastolic blood pressure, hypertensive patients with unstable angina showed an increased variability of 24-hour (p <0.01) and day-time (p < 0.05) systolic and disatolic blood pressure, together with higher values of all haemorhelogical parameters (plasma viscosity, fibrinogen and haematocrit) (p < 0.01) and beta-thrombogobulin (p< 0.05), when compared with hypertensive patients with stable angina. Moreover, significant correlations between plasma viscosity and 24-hour systolic (r = 0.42, p < 0.01) and diastolic (r = 0.39, p < 0.05) blood pressure variability were shown in hypertensive patients with unstable angina. Besides, in the same patients, the haematocrit was positively correlated with 24-hour systolic blood pressure variability (r = 0.37, P < 0.05). Our data further support the relevance of rheological determinants, platelet activation and haemodynamic factors in the genesis of the high risk condition of unstable angina.