The various methods of treating the numerous types of anterior mediastinal neoplasms are undergoing refinements; however, prior to the commencement of any treatment, an accurate histologic diagnosis must be established. We conducted a retrospective analysis of biopsies performed through a parasternal anterior mediastinotomy (PAM) on 21 patients with an anterior mediastial mass suspected of being malignant by computed tomography (CT) and other clinical information. Patients with possible localized thymomas were excluded. Mediastinotomy, which enables relatively large tissue samples to be taken, was performed through the bed of the second or third costal cartilage under local anesthesia. In 19 of the 21 patients (90%), the histologic diagnosis made from the mediastinotomy biopsy was identical to the final diagnosis. Conversely, although the tissue obtained from needle biopsy was sufficient for a histologic diagnosis in 11 of 12 patients, the histologic diagnosis made from the needle biopsy was the same as the final diagnosis in only 5 of 10 patients (50%). Thus, diagnostic accuracy was significantly higher in the mediastinotomy biopsies than in the needle biopsies (P = 0.0318). Moreover, the mediastinotomy biopsy specimens revealed subtypes of lymphomas and germ cell tumors. All of the patients from whom a mediastinotomy biopsy had been taken began appropriate therapy without delay according to the histologic diagnosis. These results suggest that the PAM approach should be chosen as the preferred method of biopsy for suspected malignant anterior mediastinal tumors.