The juxtacanalicular (JXT) tissue was evaluated in 64 specimens from 36 nonglaucomatous normal persons and 28 specimens from 26 patients with primary open angle glaucoma (POAG). Morphometry was performed on more than 2000 electron micrographs taken from the entire JXT region of each of the 64 specimens studied. The concentration of three electron-dense materials (EDMs) believed to obstruct the JXT tissue in POAG was measured using precise and reproducible morphometric methods. There is a great deal of variability in the EDM concentration, but we could still measure a significant increase in EDM of about 0.2% each year in normal specimens. Specimens from patients with POAG who are younger than 40 years of age have an EDM concentration similar to that in normal specimens. After the approximate age of 40 years, a significant difference is observed in the EDM concentration between nonglaucomatous and POAG specimens. This difference represents an average increase of 23% in POAG. Such a difference is probably too small to account for the decrease in outflow facility characteristic of POAG.