Due to the physiological tissue alterations of the breast during pregnancy and lactation the diagnosis of a breast tumor by palpation, imaging and fine-needle biopsy is difficult and often unreliable. The purpose of our study was to review the value of imaging (mammography and ultrasonography) and biopsy techniques in order to work out a reliable protocol for evaluating gestational tumors. We performed a retrospective analysis of 25 patients with clinical diagnosis of gestational breast tumors during a 4-year period from 1995 to 1999. Specimens were obtained by core biopsy for histological examination. Invasive breast cancer was diagnosed in five patients, malignant lymphoma of the skin in one patient. The tumors in the remaining 19 patients proved benign. The overall accuracy was 0.91 for conventional mammography, 0.76 for conventional and panoramic ultrasound and 0.48 for color-coded sonography. Mammography as the most reliable imaging method should be used to evaluate all patients in whom ultrasonographic findings are suspicious for tumor. Radiation phobia should not prevent the performance of mammography under these circumstances. Nevertheless, all pre-operative methods of diagnosing gestational breast tumors other than core biopsy are unreliable.