AIMS: For the diagnosis of pulmonary alveolar proteinosis from bronchoalveolar lavage specimens it is normally necessary to make an ultrastructural examination. However, this is thought to be impractical for bronchoalveolar lavage specimens that have been routinely fixed in ethanol. In the present study, bronchoalveolar lavage cytology smears on slide glasses were examined directly ultrastructurally to make a diagnosis of pulmonary alveolar proteinosis. METHODS: Bronchoalveolar lavage smears from three pulmonary alveolar proteinosis patients were stained with Papanicolaou and periodic acid-Schiff (PAS) for identification of amorphous globular structures. Subsequently, they were refixed with glutaraldehyde and osmium tetroxide, and embedded in epoxy resin. Ultrathin sections were cut and examined ultrastructurally. RESULTS: Papanicolaou stained specimens from pulmonary alveolar proteinosis patients contained scattered amorphous or granular globules, 20-50 microns in diameter, which were PAS positive. Ultrastructural examination of the globules revealed multilamellated structures, characteristic of pulmonary alveolar proteinosis, in all cases. CONCLUSIONS: In general, it is thought that the morphological diagnosis of pulmonary alveolar proteinosis from bronchoalveolar lavage specimens requires both cytological and ultrastructural examination. However, the amorphous globules evident on cytology smears proved to contain multilamellated structures so that they can themselves be used as diagnostic evidence.