Histological changes in breast tissue after radiotherapy were evaluated. Changes in tissue from 17 patients previously treated for breast cancer by surgery and radiotherapy were compared with those seen in a control group treated with surgery alone. Vascular and epithelial changes were seen only in tissue from patients who had received radiotherapy and, therefore, seemed to be relatively specific; stromal changes were seen in both groups and seemed, therefore, to be non-specific. Epithelial atypia is of particular importance as it may be severe and be confused with recurrent malignancy. The presence of other changes associated with radiotherapy, particularly those in vessels, should help to avoid such misdiagnosis.