71 patients with recurrent syncope which remained unexplained after standard clinical and electrophysiological investigation underwent 60 degree head-up tilt. This procedure reproduced symptoms with vasovagal syncope in 53 (74%), 40 of whom had bradycardia, some with prolonged asystole, during syncope. The other 13 patients had predominant vasodepression with hypotension. Mean time to syncope after tilt was 25 min. Patients with conduction tissue disease and age-matched control subjects had a 15% and 7% incidence of tilt syncope, respectively. Temporary dual-chamber pacing aborted syncope in 85% of subjects, and improved cardiac index and systemic blood pressure during tilt. Long-term results indicate that selected patients may benefit from permanent dual-chamber pacing. Head-up tilt is useful in the investigation of unexplained syncope because symptoms are reproduced in front of a medical witness.