Histopathological, ultrastructural and polyacrylamide gel electrophoretic examinations were carried out on biopsy, cyst fluid, surgical pathology and autopsy specimens obtained from 7 cystic craniopharyngioma cases before and after yttrium-90 silicate colloid (90Y) irradiation. Light microscopy revealed that the lining epithelial tumor cell layer of the cyst wall was destroyed, and scar tissue containing large amount of hyaline degenerated collagen bundles replaced it. Proliferative postirradiation vasculopathy was also demonstrated in the cyst wall following 90Y installation. Electrophoretic property of cyst fluid was similar to the normal human serum. Considering that scar tissue has a certain propensity to shrink, the fibrosis in the cyst wall together with destruction of neoplastic epithelium and vascular changes might explain diminished fluid production and cyst volume reduction after 90Y treatment.