Late potentials are depolarizations which arise from areas of delayed ventricular activation and may indicate a propensity for ventricular tachycardia. Sixty-four subjects were assessed by non-invasive measurement. Late potentials were not present in 20 subjects with normal hearts nor in 6 patients with cardiac disease but with no evidence of ventricular tachycardia (VT). Seventeen of 20 patients with recurrent sustained ventricular tachycardia (RSVT) and 2 of 10 patients with unsustained VT had late potentials. None of the 6 patients with automatic VT or the 2 patients with torsades de pointe had late potentials. In a subgroup of 28 symptomatic patients in whom programmed ventricular stimulation was performed, late potentials correlated with inducibility of sustained VT (P less than 0.05). Late potentials may therefore serve as a useful marker of RSVT and confirm a re-entrant mechanism of VT.