Purpose The cardiac and antineoplastic effects of trastuzumab may be related to specific uptake of trastuzumab in myocardium and tumor tissue, respectively. We evaluated whether indium-111 (In-111) -labeled trastuzumab scintigraphy can predict cardiotoxicity and identify tumor lesions. In addition, we evaluated whether plasma markers for cardiac dysfunction can be used to predict cardiotoxicity. Patients and Methods Patients with human epidermal growth factor receptor 2 (HER2) -positive metastatic breast cancer underwent gamma camera imaging from 15 minutes to 7 days after injection of 150 MBq In-111-diethylenetriaminepenta-acetic acid anhydride (DTPA) -trastuzumab, after loading-dose trastuzumab, and after once-a-week trastuzumab doses for 11 weeks, and concomitant paclitaxel once every 3 weeks. Cardiac assessments were performed before treatment, and after four and six cycles. Plasma N-terminal probrain natriuretic peptide (NT-proBNP) and serum troponin I were measured with immunoassay. Results Fifteen of the 17 patients were available for cardiac and tumor uptake analysis. On the first scan, myocardial In-111-DTPA-trastuzumab uptake was observed in one patient with pre-existing cardiac arrhythmias, who did not develop heart failure during treatment. Severe cardiotoxicity occurred in three patients, without initial myocardial uptake, whereas one showed weak myocardial uptake after four cycles. The detection rate of single tumor lesions was 45%. New tumor lesions were discovered in 13 of 15 patients. Pretreatment plasma NT-proBNP levels were higher in patients with than without heart failure (mean, 534 [standard deviation, 2361 v 105 [standard deviation, 791 ng/L; P =.009). Conclusion Radiolabeled trastuzumab scintigraphy was not valuable in predicting trastuzumab-related cardiotoxicity in metastatic breast cancer patients, but can identify HER2-positive tumors. Measurement of plasma NT-proBNP is promising regarding prediction of trastuzumab-related cardiotoxicity.