Histopathologic analysis of an anterior mediastinal mass of unknown origin is essential for treatment decision. Mediastinoscopy is the most common procedure performed to obtain biopsies, but general anaesthesia and hospitalization are necessary. The aim of this study was to evaluate whether transsternal core biopsies, an easy outpatient biopsy technique, could be an alternative to mediastinoscopy. A biopsy instrument that makes it possible to reach tumours hidden behind bone was used for transsternal CT-guided core biopsies in 21 patients with a newly diagnosed anterior mediastinal mass. No severe side effects were observed. In 19/21 (90%) patients the biopsies were diagnostic. In 2/21 patients additional biopsy techniques had to be used. In these two patients Hodgkin's disease was suspected in the first biopsy procedures. The diagnosis was confirmed by new core biopsies, from other parts of the tumour, not using a transsternal approach (transclavicular and parasternal, respectively). In addition, one mediastinoscopy was performed in a patient who was diagnosed with a non-Hodgkin's lymphoma but where more material was needed for lymphoma subclassification. It is concluded that CT-guided transsternal core biopsy is a clinically valuable method in patients with a newly diagnosed anterior mediastinal mass.