The objective of this study was to determine the effect of intravenous haloperidol on QT interval dispersion in critically ill patients and to compare increases in QT interval dispersion and QTc intervals in patients who developed haloperidol-induced Torsades de Pointes versus those in patients who did not. This was a case-controlled study of 30 critically ill patients who received intravenous haloperidol for delusional agitation. Cases were patients (n = 6) who developed Torsades de Pointes during haloperidol therapy. Controls were patients (n = 24) who did not experience haloperidol-induced Torsades dePointes. QTc intervals were measured and QT interval dispersion was calculated. Haloperidol prolonged QTc interval compared to pretreatment values in Torsades de Pointes patients (606 +/- 61 ms vs. 501 +/- 44 ms, p = 0.007) by a greater magnitude than in patients who did not experience Torsades de Pointes (507 +/- 60 ms vs. 466 +/- 44, p = 0.01). Twelve-lead analysis revealed that QT interval dispersion increased in patients who experienced Torsades de Pointes (from 63 +/- 11 to 95 +/- 22 ms, p = 0.03) but not in those who did not (62 +/- 18 vs. 60 +/- 26 ms, p = 0.66). Analysis of precordial leads only showed no significant haloperidol-associated increases in QTinterval dispersion in eithergroup. The odds of developing haloperidol-induced Torsades de Pointes were highest in patients with QTc interval > 521 ms during haloperidol therapy(odds ratio = 12.1). It was concluded that intravenous haloperidol prolongs QTc intervals in critically ill patients. The degree of prolongation is greater in patients who experience Torsades de Pointes. QT interval dispersion may be increased in patients who develop haloperidol-induced Torsades de Pointes compared with those who do not. However, these effects are dependent on the method of measurement (12 leads vs. precordial leads). In addition, the odds of haloperidol-induced Torsades de Pointes are higherin patients with QTc intervalprolongation compared with increased QT interval dispersion. Therefore, QTc interval determination remains preferable to QT interval dispersion as a means assessment of risk for haloperidol-induced Torsades de Pointes.