From a study of tissue from 34 cancerous breasts and 34 benign breasts the nature and occurrence of radial scars has been assessed. The frequency of detection was influenced more by the amount of tissue assessed and by the diligence of search than by any association with cancer. Bilaterality and multifocality were features in cancerous and benign breasts and a similar range of scar histological appearance was apparent in both types. The histological features of 103 radial scars were tabulated to compare the association of characteristics within individual scars, and the information used to formulate modes of development and progression. The 'early' stage appeared to correlate with central chronic inflammatory response, many spindle cells and minor fibroelastotic distortion of parenchyma; the 'late' stage showed few spindle cells and dominant fibroelastosis with considerable distortion and separation of parenchymal elements. No support was found for the concept that the lesion is premalignant. Rather it belongs to the spectrum of sclerosing alterations affecting the breast parenchyma which ranges from physiological through to clearly pathological changes. Position in the spectrum depends upon the extent and degree of change.