Doxorubicin is an anthracycline antibiotic with a broad spectrum of antineoplastic activity. Cardiotoxicity is a serious long-term complication of the drug. Simultaneous administration of carnitine has been proposed to prevent cardiotoxicity. We aimed to monitor the serum carnitine levels during the treatment of doxonibicin and to determine a relationship between serum carnitine levels and cardiac dysfunction. Fifteen patients were evaluated prospectively. Measurement of carnitine levels and evaluation of cardiac function were performed prior to treatment, and after cumulative doses of 180 and 300 mg/m(2) of doxorubicin. A group of 20 healthy children served as control group to obtain reference values. We found subclinical abnormalities in cardiac function, while the cumulative doses of the doxorubicin was increasing. The mean end diastolic and end systolic left ventricular dimensions of the patient group after completion of the treatment were significantly increased compared with initial values. The ejection and shortening fraction of the patient group after cumulative doses of 300 mg/m(2) of doxorubicin were significantly lower than those of the control group. A statistically significant augmentation was observed in mitral A, with a decrease in mitral E/A ratio. There was a trend towards lower serum carnitine levels with higher cumulative doses of doxorubicin, although it was not statistically significant. Our results invite new detailed investigations depending on the measurement of serum and urinary free and acyl carnitine and myocardial carnitine levels to evaluate possible roles of carnitine in the prevention of doxorubicin-induced cardiotoxicity.