Microdochectomy for persistent discharge from a single lactiferous duct was performed in 162 women. Invasive or in situ ductal carcinoma was diagnosed in 16 patients (10 per cent), none of whom had a palpable lump. The discharge was blood-stained in 14 of these women and in two it was clear. Mammography was performed in 15 of these 16 patients and was abnormal in only five. Three patients had atypical ductal hyperplasia, one of whom subsequently developed an invasive ductal carcinoma. Microdochectomy for persistent discharge from a single lactiferous duct is curative and gives a diagnosis of the cause. It remains the treatment of choice whether the discharge is blood-stained or clear.