Twenty-one patients with primary open-angle glaucoma were followed an average of 4.4 years after developing monocular field loss. Nine of the undamaged fellow eyes developed field loss, while 16 of the 21 eyes presenting with field loss showed progression of field changes. In 13 patients, the presenting eye had a greater progression of field loss than the fellow eye, and asymmetry of pressure control explained this in only five patients. Eleven patients with symmetric pressure control had progressive field changes in one or both eyes, and in seven, the presenting eye showed greater progression than its fellow. This suggests that an eye with field loss may be more susceptible to progression of field changes at similar pressure levels than an eye without previous damage. Furthermore, patients with unilateral field loss have a greater incidence of field loss in the undamaged fellow eye than do patients with bilateral ocular hypertension.