Objective To investigate the manifestations and influential factors for acute radiation-induced heart damage (RIHD) after three-dimensional radiotherapy in patients with thoracic neoplasms. Methods Three-dimensional conformal radiotherapy or intensity-modulated radiotherapy was delivered to 54 patients with thoracic neoplasms. The status of acute RIHD was evaluated by Common Terminology Criteria for Adverse Events, Version 3.0. The associations of manifestations, basic cardiac function, and clinical and physical factors with acute RIHD were analyzed. A paired t-test was used for comparison between groups;a logistic regression analysis was used for multivariate and univariate analysis. Results The overall incidence rate of acute RIHD was 70%(38 patients), all of grade 1. Of the 38 acute RIHD patients, 37 had abnormal electrocardiogram (ECG), 7 had elevation of myocardial enzymes, 7 had decrease in left ventricular systolic function, 6 had cardiac troponin I increase, and 3 had pericardial effusion. Of the 37 patients with abnormal ECG, 19 had ST-segment changes, and 12 had sinus tachycardia. None of the basic cardiac function, history of smoking, history of chemotherapy, and history of heart disease was associated with acute RIHD (χ²=0—1.98, P=0.160—0.999), according to the univariate analysis. The multivariate analysis revealed that the relative volume of the heart exposed to 60 Gy irradiation (V₆₀) was the independent influential factor for acute RIHD (χ²=5.10, P=0.024). Conclusions Acute RIHD is less severe after three-dimensional radiotherapy than after conventional radiotherapy, andabnormal ECG is the most common. V₆₀ of the heart is the independent influential factor for acute RIHD.