A retrospective analysis of 40 patients with pleural effusions caused by breast cancer is presented. Evidence is presented which suggests that malignant pleural effusion in breast cancer is caused by lymphatic rather than blood spread. 70% of effusions are ipsilateral to the primary carcinoma and 22.5% of patients with pleural effusions have positive internal mammary node biopsies (compared to 10.4% in controls). 47.5% of effusions had positive cytology but this does not correlate with either survival or recurrence. In operable tumours, pleural effusion is correlated with the size of the primary tumour but effusions were uncommon with large, locally advanced Stage III tumours at presentation, which could be attributable either to the short survival of such patients or to the effect of adjuvant radiotherapy reducing the chance of an ipsilateral effusion. Although these patients overall had a short life expectancy (median 11 months) there was a wide range. Chest drainage and instillation of 100 mg mepacrine in 30 ml of bupivacaine to give rise to pleurodesis offers useful palliation and minimizes recurrence (29 treatments with only two symptomatic recurrences), compared with aspiration alone (19 treatments with 12 symptomatic recurrences) or instillation with tetracycline (10 treatments with five symptomatic recurrences). Pleural effusion secondary to breast cancer is a locoregional manifestation. Pleurodesis with mepacrine offers good local control.