Pulmonary alveolar proteinosis (PAP), a disease characterised by accumulation of surfactant in alveoli, is diagnosed on the basis of invasive biopsy procedures. We have measured apoprotein A (SP-A) concentrations in sputum to see if this is useful for the diagnosis of PAP. Sputum samples from three patients with PAP and twenty patients with other pulmonary disease were assayed using monoclonal antibodies to SP-A. SP-A concentrations were 400 times higher in patients with PAP than in the controls, suggestions that this measurement is useful for the diagnosis of PAP especially where lung biopsy is contraindicated.