The reported therapeutic range for trough flecainide concentration is 200-1000 ng mL(-1). Severe adverse events, such as ventricular arrhythmias, have occurred occasionally in patients whose serum flecainide exceeded 1000 ng mL(-1). However, the lower limit remains controversial. We have evaluated blood flecainide concentrations in patients with tachyarrhythmia who received the drug to control palpitation. We measured the flecainide trough levels and incidence and frequency of palpitation of 44 outpatients receiving oral flecainide (150-300 mg daily). Mean serum flecainide trough concentrations differed significantly between patients with (n = 14) and without (n = 30) palpitation (259.5 +/- 85.2 vs 462.2 +/- 197.7 ng mL(-1), P < 0.01). The frequency of palpitation decreased as the serum flecainide concentration increased. The incidence of palpitation was 65% at serum flecainide concentrations < 300 ng mL(-1) and 11% at > or = 300 ng mL(-1). QRS values were increased significantly in patients with serum flecainide < 300 ng mL(-1) compared with > or = 300 ng mL(-1) (0.110 +/- 0.016 s vs 0.093 +/- 0.019 s, P < 0.05). We concluded that to control paroxysm in patients receiving flecainide for tachyarrhythmia serum flecainide concentrations should be maintained at > or = 300 ng mL(-1).