Head-up tilt, a recognised stimulus to vasovagal syncope, was used to investigate syncope that remained unexplained despite full clinical and electrophysiological assessment in fifteen patients, mean age 65 +/- 10 years, who had had 15 +/- 19 episodes of unexplained syncope over periods of a week to 26 years. After overnight fast systolic blood pressure and heart rate were continuously monitored during 40 degrees head-up tilt for 60 min. Ten control subjects with no history of syncope were studied similarly. In ten patients (67%) and one control vasovagal syncope developed after 29 +/- 19 min (p less than 0.001). In symptomatic patients systolic blood pressure fell from 150 +/- 32 to 56 +/- 9 mm Hg (p less than 0.001) and heart rate from 62 +/- 9 to 38 +/- 12 beats per min (p less than 0.01). In each case symptoms during the test reproduced those previously experienced. No clinical findings predicted development of syncope during tilt. Baseline systolic blood pressure and heart rate did not differ significantly between patients and controls. Pacemakers were implanted in seven patients who have remained symptom-free since implant (follow-up 10 +/- 3 mo).