The smoking habits of 51 patients with glomerulonephritis caused by autoantibodies to the glomerular basement membrane (GBM) were investigated. 43 patients also had pulmonary haemorrhage; 8 did not. A full smoking history was obtained from 47 patients. Of these, all 37 cigarette smokers suffered lung haemorrhage, compared with only 2 of 10 non-smokers. There was no significant difference between the titres of circulating anti-GBM antibodies in smokers and non-smokers. In 1 patient resumption of smoking was closely followed by recrudescence of lung haemorrhage. These results largely explain the variable presence of pulmonary haemorrhage in anti-GBM disease.