M-mode indices of left ventricular dimension and posterior wall thickness were derived from echocardiograms of children presenting with dilated cardiomyopathy/myocarditis and were related to outcome. Echocardiograms from 16 of 18 children were manually digitized to obtain changes of left ventricular dimension and posterior wall thickness throughout the cardiac cycle. Indices of ventricular function and the ratio of end-diastolic posterior wall thickness to cavity dimensions were obtained. Patients were divided into group I (alive, n = 7), and group II (died, n = 6 or heart transplantation, n = 3) at median follow-up of 25 months. No significant difference was seen for the shortening fraction, the percentage of posterior wall thickening or the normalized peak rate of left ventricular filling. The normalized peak rate of posterior wall thinning was greater in group II. The posterior wall thickness to cavity dimension ratio was higher in group I (median = 0.19) than group II (median = 0.13) (P < 0.005). Five of six survivors, whose ventricular function improved, had ratios > 0.17. All but one with a ratio < or = 0.16 remained with a dilated heart, died or required transplantation (P = < 0.01). A relatively thicker posterior wall (ratio > 0.17) was associated with better prognosis and recovery. This index should be taken into account in decision-making regarding timing for cardiac transplantation.