We have developed a device capable of producing subsurface images in eyes at microscopic resolution. We call this method "ultrasound biomicroscopy." We examined 19 patients with pigmentary glaucoma or pigmentary dispersion syndrome using ultrasound biomicroscopy. Ten patients were found to have concave irides with small distances between the back of the iris and the zonule. In these patients iris-lens contact was greater than in patients without iris concavity and healthy patients. Nine patients did not show iris concavity or increased iris-lens contact. The finding of iris concavity and increased iris-lens contact is compatible with a theory of reverse pupil block producing intermittent iris-zonule touch, with a valve effect preventing posterior flow of aqueous. This theory was supported by the loss of concavity following iridotomy in three patients. The force that produces a reversal of the normal pressure gradient remains unclear, but recent evidence suggests that accommodation plays a major role. The patients who did not show iris concavity and increased iris-lens contact may have been examined at a time when these forces were not acting or had ceased to act.