The efficacy of flecainide acetate in suppressing ventricular premature contractions in 14 patients was evaluated in a double-blind, cross-over, placebo controlled, randomized and balanced study. Each treatment period was 2 weeks followed by a 3-4 d placebo washout period and the study lasted 5 weeks. Flecainide was given in a dose of 200 mg twice daily. A significant reduction in the total number of QRS complexes in a 24 h period was observed during the active compared with placebo treatment (P less than 0.05). In comparison with placebo, flecainide reduced the number of aberrant and premature aberrant QRS complexes (P less than 0.01). The mean suppression rate of aberrant QRS complexes during flecainide treatment was 85.4% and that of premature aberrant complexes was 93.2%. Maximum heart rate measured by 24 h ECG decreased significantly during flecainide therapy (P less than 0.01), although no change occurred with resting heart rate measured clinically or by ECG. Severe dizziness associated with flecainide therapy necessitated withdrawal of 2 patients from the study. A proarrhythmic effect of flecainide, ventricular tachycardia, was observed in one patient.