The results of a well-controlled multicenter shortterm safety and efficacy study, supported by results from several long-term studies, indicate that therapeutic doses of flecainide are well tolerated by most patients. The most frequently reported extracardiac adverse experiences were dizziness (30%) and visual disturbances (28%), often occurring in tandem. Headache, nausea, dyspnea and chest pain occurred at incidences of 6 to 9%; other adverse experiences occurred at incidences of greater than or equal to 5%. Because of study design, it is likely that these figures are overestimates; they include all reports, whether or not they were caused by flecainide. Extracardiac adverse experiences were given as reasons contributing to discontinuation of therapy in 10% of patients in the short-term and 6% of patients in the long-term studies. In most cases the inability to tolerate flecainide became evident early in therapy. No new adverse experiences indicative of any chronic toxic effect of flecainide were reported during the long-term studies. Side effects tended to be intermittent and to decrease over time.