The visual fields of 54 eyes (27 patients) having a medical diagnosis of open-angle glaucoma and receiving treatment were compared with the visual fields recorded at the time of diagnosis of the disease. Visual fields remained stable in 34 (62%) of the glaucomatous eyes. However, reduction of the intraocular pressure (IOP) did not prevent the progression of existing field defects in 16 of 21 eyes nor prevent field damage from occurring in 4 of 33 eyes which had normal visual fields at the commencement of treatment. The presence of a visual field defect at the time of diagnosis was clearly associated with a high risk of continued field decay during treatment. Intraocular pressure at the time of diagnosis did not identify which eyes would develop visual field loss or suffer further loss despite hypotensive treatment. Likewise, the extent to which the IOP was reduced by treatment did not appear to be related to the decay of visual field.