Summary—The relationships between plasma mexiletine levels and the presence of ventricular arrhythmias and side effects were studied on patients from IMPACT (International Mexiletine and Placebo Antiarrhythmic Coronary Trial). 630 patients who had suffered a myocardial infarction were randomized between placebo and mexiletine.Plasma levels were measured 1 month after the beginning of treatment. Arrhythmia findings (presence or absence of premature ventricular contractions (PVCs), couplets, runs, bigeminy, trigeminy, multiformity and various combinations) were assessed from 24‐h ambulatory ECG recordings. The empirical logistic transform was used for modeling the relationships. For all these variables, except the presence of PVCs, trigeminy and runs, the association with plasma level was significant: the higher the plasma level, the lower the rate of occurrence of the particular arrhythmia. This was true whether or not the patient had the arrhythmia at baseline. Analyses based on the same model showed a significant correlation between plasma level and tremor, constipation, sexual problems and the presence of at least one side effect.As the levels of mexiletine at which side effects become frequent are in the same range as those necessary to suppress arrhythmias, the therapeutic range is narrow and individual dose adjustment should preferably be m