Three patients who developed pulmonary fibrosis following prolonged treatment with nitrofurantoin for chronic urinary tract infections are presented. They had received the drug for 3--4 years; all gave histories of 2--3 years of progressive shortness of breath and an unproductive cough. On examination, all 3 patients had bilateral crackles and 1 had finger clubbing. Chest radiographs showed bilateral shadowing and lung function tests a restrictive defect with reduced gas transfer. Lung biopsies showed extensive fibrosis in 2 patients and advanced honeycomb formation in 1. The response to steroid therapy in 2 patients was excellent, with resolution of symptoms, clearing of the chest radiographs and improvement in lung function. It is proposed that nitrofurantoin has no place in the long-term treatment of chronic urinary tract infections and that its use in acute infections should be questioned.