To investigate the pattern and prevalence of membranous fat necrosis of the breast we examined 25 breast biopsies or mastectomy specimens performed for recurrent lumps following conservative surgery and radiation therapy. In addition, we examined 41 consecutive cases taken from our files coded as fat necrosis which were associated with several aetiologies. Our results demonstrate that membranous fat necrosis of the breast occurs in association with several aetiologies but in its florid form is most often due to radiation therapy.