A total of 108 patients with mammary duct ectasia have been reviewed. Patients who presented with breast pain or a lump were significantly younger than those with nipple retraction. A histological review of the 88 patients who underwent biopsy showed severe periductal inflammation around non-dilated ducts occurred in younger patients and was associated with the symptom of breast pain or a lump, whereas older patients had duct dilatation as the most prominent feature, frequently associated with nipple retraction. These findings suggest periductal mastitis precedes duct dilatation and is the initial disorder in this condition. Data on parity and breast feeding for the 108 patients with duct ectasia were compared with age-matched groups of 505 patients with breast cancer, 199 with fibroadenoma, 240 with cysts and 912 'normals'. There were no differences in parity or breast-feeding history between the patients with duct ectasia and any other group, indicating that neither parity nor breast feeding is an important aetiological factor in this condition.