Purpose: To evaluate a semiautomatic gun to retrieve diagnostic core specimens of lesions in the breast and the axillary region. Material and Methods: In a series of 180 consecutive core biopsies (2.1 mm), 145 (142 breasts and 3 axillae) were performed with a semiautomatic gun (18 stereotaxic and 127 US-guided) from lesions presenting mammographically as microcalcifications (n = 15) and opacities (n = 130). The gun did not work satisfactorily in 34 lesions, which were tumors with a very hard consistency. One additional patient was excluded because of technical failure. Biopsy diagnoses in the 145 patients were correlated to surgical histopathology, follow-up mammograms and/or clinical findings. Results: Histologic examination of the specimens resulted in correct diagnoses in 89% (129/145) of the total material and in 87% (108/124) of cancers. In 107 cases, in which only 1 specimen was obtained, 83 of 89 cancers (87%) were detected. Length of specimens ranged from 3 mm (n = 2) to 17 mm (n = 31). Among patients with a 17-mm-long specimen, there was only 1 false-negative diagnosis. Conclusion: The semiautomatic gun provided diagnostic specimens in a majority of cases and could be used as an alternative to the automatic guns when size or location of the lesions necessitates a high precision. It was not suitable for use in very hard tumors.