Pulmonary alveolar proteinosis (PAP) is a rare disorder characterised histologically by an intra-alveolar accumulation of fine granular eosinophilic and periodic acid-Schiff positive material. In a retrospective study, the composition of the intra-alveolarly accumulated material of adult patients with PAP was analysed by means of immunohistochemistry and Western blotting. In patients with PAP, the current authors found an intra-alveolar accumulation of surfactant protein (SP)-A, precursors of SP-B, SP-B, variable amounts of mono-, di-, and oligomeric SP-C forms, as well as SP-D. Only in one patient was a precursor of SP-C detected. By means of immuno-electron microscopy, the current authors identified not only transport vesicles labelled for precursors of SP-B and SP-C, but also transport vesicles containing either precursors of SP-B or SP-C in type-II pneumocytes in normal human lungs. It is concluded that pulmonary alveolar proteinosis in adults is characterised by an intra-alveolar accumulation of surfactant protein A, precursors of surfactant protein B, and surfactant proteins B, C and D. The current data provide evidence that not only an impairment of surfactant clearance by alveolar macrophages, but also an abnormal secretion of transport vesicles containing precursors of surfactant protein B (but not surfactant protein C) and an insufficient palmitoylation of surfactant protein C, which may lead to the formation of di- and oligomeric surfactant protein C forms, play a role in the pathogenesis of pulmonary alveolar proteinosis.