Haemosiderin-laden alveolar macrophages are a common finding in patients with alveolar bleeding. Iron-positive macrophages, suggestive of subclinical alveolar bleeding, were found to be fairly common in bronchoalveolar lavage (BAL) fluid in primary systemic vasculitis but uncommon in collagen vascular diseases (CVDs) and rheumatoid arthritis (RA). To substantiate the impression that subclinical alveolar bleeding may be a feature distinguishing between these disorders, fibreoptic bronchoscopy and BAL were performed in 49 patients with active Wegener's granulomatosis or Churg-Strauss syndrome and 44 patients with CVDs or RA, all of them without clinically manifest alveolar bleeding. The percentage of iron-positive cells was compared with clinical and radiological findings. Only a minority of the CVD and RA patients had iron-positive alveolar macrophages; the 95th percentile of the median number of such cells was 5%. Fifty-three per cent of the patients in the vasculitis group had >5% iron-positive cells, with individual counts ranging up to 95%. Patients with iron-positive macrophages had more extensive disease, more frequent microhaematuria, a higher antineutrophil cytoplasmic antibody titre, a higher myeloperoxidase concentration in the BAL fluid and somewhat more frequent low-attenuation opacities in pulmonary high-resolution computed tomography than the patients with a low iron-positive cell count. In conclusion, subclinical alveolar bleeding was, indeed, a common finding in antineutrophil cytoplasmic antibody-associated vasculitis, which distinguished these disorders from lung disease due to collagen vascular diseases or rheumatoid arthritis. Its association with indices of disease activity, although weak in this cross-sectional study, merits a longitudinal study of its value for the long-term monitoring of vasculitis patients.