Amiodarone hydrochloride is increasingly being used in the treatment of ventricular and supraventricular arrhythmias. Although a highly effective anti-arrhythmic agent, its use is restricted by the high incidence of side effects. To elucidate the value of monitoring serum level of both the parent drug and its active metabolite in predicting the occurrence of side effects, the investigators examined 109 patients from a register of patients treated with amiodarone for the prevalence of known side effects of the drug. The register contained over 90% of patients treated with amiodarone at the Leicester General Hospital during the period of the study. The findings suggest cutaneous side effects and abnormal thyroid function tests (without overt gland dysfunction) are more likely to occur with increasing duration of treatment and cumulative dosage. However, neither the serum amiodarone level nor the serum metabolite level had any predictive power for the occurrence of side effects. In view of this finding, it is recommended that close attention be paid to the continued clinical monitoring of side effects and that there is utility in measuring the serum amiodarone level in each patient to avoid the prescription of unnecessarily high doses. This is necessary not only to lessen the occurrence of cumulative dose-related side effects, but also because the variable but very long half-life of the drug leads to difficulties in relating spot drug levels to long-term effects.