Tissues from six functioning and four failed filtering blebs in glaucomatous eyes examined by light and electron microscopy showed normal epithelium. The response of the subepithelial connective tissue differed in the two groups. Failed blebs had dense collagenous connective tissue in their walls. In functioning blebs, the subepithelial connective tissue was loosely arranged and contained histologically clear spaces. These clear spaces corresponded in size and position to microcystic spaces seen clinically in functioning blebs. The presence of microcystic spaces visible by slit-lamp examination is probably a good sign of bleb function.