We examined the clinical characteristics and electrocardiographic findings of 7 patients having the acquired long QT syndrome who developed torsades de pointes while receiving no antiarrhythmic drugs. A total of 43 episodes of torsades de pointes were documented among these patients. Underlying heart diseases were present in 6 patients and hypopotassemia (< or = 3.3 mEq/l) in 4. Four had bradycardia (< or = 52 beats/min) immediately before the development of torsades de pointes. The QTc intervals measured immediately before the episodes of torsades de pointes were significantly longer than those 6-24 hours before the episodes (0.69 +/- 0.10 vs 0.56 +/- 0.10 sec, p < 0.05), while heart rates did not differ significantly between these 2 periods (54 +/- 12 vs 58 +/- 15 beats/min). The ventricular rate of torsades de pointes was 192 +/- 24 beats/min. A "long-short initiating cycle" was noted in all 43 episodes, and the initiating premature ventricular beat (PVB) showed the "R on T(U)" phenomenon in 42 of the episodes. A notched T-U complex due to a prominent slow wave (U wave) at the end of the T wave was noted in 5 patients immediately before the episodes of torsades de pointes. Prolongation of the preceding RR interval was directly related to the increase of the U wave amplitude, which caused an increased likelihood of the occurrence of PVBs near the peak of the U wave. Torsades de pointes developed from the largest U wave. Direct current cardioversion was transiently effective for treating torsades de pointes, and intravenous lidocaine, atropine and verapamil were effective in some cases.(ABSTRACT TRUNCATED AT 250 WORDS)