Thirty-one trabeculectomy specimens of patients suffering from primary open-angle glaucoma (POAG) who had received no, or only minor, medication prior to surgery were ultrastructurally and quantitatively analysed. Most of the specimens revealed thickened trabeculae, increased amounts of plaque-material deposited within the cribriform layer and an abundance of long spacing (lattice) collagen. The uveal meshwork was partly deprived of cells whereas the cribriform layer often contained numerous enlarged, light cells with many small mitochondria and lysosomes but no prominent endoplasmic reticulum or Golgi complexes. The quantitation of sheath-deprived plaque material (SD plaques) in a defined area of inner and outer wall of Schlemm's canal showed no significant difference between the untreated cases studied here and the treated ones studied previously. However, both groups had significantly higher amounts of SD plaque material in the inner wall than normal controls of a similar age range.