This report illustrates the use of cardiac magnetic resonance imaging (MRI) to quantify the initial extent of a cardiac rhabdomyosarcoma and, more importantly, its response to chemotherapy. Image slices spanning the heart and adjacent structures were analyzed using Simpson's rule applied to the image slices to estimate the tumor volume initially, then after 5 weeks, and again after 4 months of chemotherapy. A substantial, progressive reduction in tumor volume during chemotherapy was shown. After chemotherapy was discontinued, an increase in tumor volume was shown. It is suggested that, in addition to being useful in patient care, the technique may be useful in clinical investigations by providing an objective, quantitative measure of tumor response to therapy.