To investigate the fine-structural nature of nuclear inclusions immunopositive for surfactant protein A (SP-A) antibody staining, a detailed ultrastructural study was performed, as well as immunohistochemical examination of pulmonary adenocarcinomas. Surgically resected tumor specimens from 31 patients were examined by immunohistochemistry focused on reactivity to SP-A and thyroid transcription factor 1 (TTF-1) antibodies. Only cases with >5% positive nuclear inclusions in cancer cells were considered positive, some of which were examined by electron microscopy. Immunohistochemically, 6 of 31 cases were doubly positive for SP-A and TTF-1 antibodies. On electron microscopy, SP-A-positive nuclei contained diffuse or globular fine granular substance as inclusions. Both types of globular and diffuse inclusions were sometimes connected to the inner nuclear membrane, in association with fragmented or stacked membranous structures. The findings of this study suggested that nuclear inclusions positive for SP-A antibody staining in adenocarcinomas of the lung were derived from accumulated content in the perinuclear cistern resembling pseudoinclusion processes and composed of proteins antigenically cross-reactive with SP-A.