Three cases of breast cancer treated at Memorial Cancer Center in Warsaw developed in the wall of bening cysts. All cases were submitted to the routine diagnostic procedures: clinical examination, X-ray mammography, ultrasonography and fine needle biopsy. In 2 cases the performed procedures did not manage to identify the malignant character of the lesions. That is why we suggest to extend the routine diagnostic performances with the pneumocystomammography in future. (author). 6 refs, 2 figs.