The number of survivors of childhood cancer affected by anthracycline cardiomyopathy is steadily increasing, despite efforts to limit cardiotoxicity by dose restriction. Cardiac function was evaluated prospectively in 125 children during treatment to attempt to identify individual susceptibilities to cardiotoxicity and hence any potential for treatment modification. Left ventricular shortening fraction was used as an index of cardiotoxicity. Shortening fraction declined as cumulative anthracycline dose increased, at an average rate of 1% per 100 mg/m2. Six patients (5%) developed heart failure. Twenty four patients (19%) had abnormal shortening fraction (< 30%) by the end of treatment, and their rate of fall of shortening fraction was significantly steeper throughout treatment than in patients finishing with normal function (shortening fraction > or = 30%). This differential susceptibility to cardiotoxicity was apparent from very early in treatment, interquartile ranges of the two shortening fraction groups separating at doses > 200 mg/m2. Patients at high risk of risk of important anthracycline cardiotoxicity may be identifiable early in treatment by regular and careful monitoring of shortening fraction. However, frequent assessment is required and this has significant resource implications.