A total of 41 eyes of 28 patients with primary open-angle glaucoma and low-tension glaucoma (group 1) were followed for 12-72 months with tonometry and automated perimetry. A second group consisting of 13 eyes (9 patients) with secondary glaucoma (pigment dispersion, goniodysgenesis, ciliolenticular block and chronic angle closure) was followed for 13-60 months. None of the eyes had undergone filtering surgery during the follow-up period. The visual field decay was calculated by means of regression analysis of the mean defect. There was a significant correlation between visual field decay and both mean intraocular pressure (IOP, P > 95%) and maximal IOP (P > 98%) in the secondary glaucomas, which was not found in group 1. IOP oscillation expressed by the standard deviation of IOP was significantly correlated with progressive visual field loss in both groups. The slope of visual field decay was independent of the functional damage at the initial examination. Even if the influence of IOP level on the course of primary chronic glaucoma may have been masked by other factors that have a great interindividual variation, the oscillation of IOP seems to be at least as important as the level of IOP.