Orthostatic tolerance in 79 patients complaining of attacks of unexplained syncope, was assessed as the time to imminent syncope in a test involving: head-up tilt by 60 degrees for 20 min, followed by tilt and lower body suction at -20 and -40 mmHg for 10 min at each. Blood pressure and heart rate were determined noninvasively. Ninety-five per cent of patients developed signs of presyncope during the test. After 10 min of lower body suction at -20 mmHg, presyncope had occurred in 85% of the patients compared with only 23% in a recently reported group of asymptomatic controls. Both patients and controls were divided into four groups: men and women, under and over 50 years, and the times at which each group of control subjects showed a 20% incidence of syncope were taken as the limits of normality. By those times, overall 85% of patients had developed syncope. It is concluded that the new combined test is able to discriminate patients who have poor orthostatic tolerance and is likely to be of value in assessing the effects of treatment regimes.