We have studied 27 patients on long-term treatment (6-60 months) with amiodarone (dose range 350 mg per week to 2800 mg per week) to ascertain whether the corrected QT interval could predict plasma amiodarone or plasma desethylamiodarone concentration. The patients were assessed on three or four occasions one month apart. There were positive correlations for delta % QTc and plasma amiodarone and delta % QTc and plasma desethylamiodarone. There was approximately a four-fold variation for delta % QTc and plasma amiodarone. This variation was not accounted for by between-occasion variation in the QTc interval, as the coefficient of variation was 2.1%. In six of the patients delta % QTc either decreased or did not change. delta % QTc is not a reliable predictor of plasma amiodarone concentration in the individual patient over time.