It is generally assumed that successful filtering surgery, by maintaining intraocular pressure at a low level, will protect a glaucoma patient's remaining visual field. We present three patients with chronic open angle glaucoma and typical visual field changes. In each case, filtering surgery was performed because of progressive loss of visual filed and inadequate pressure control. Despite excellent pressure levels after operation, field loss continued in the operated eye. Certain genetic and systemic factors may contribute to further loss of visual field in the presence of normal intraocular pressures. We stress the importance of careful and continuous examination of the visual fields in all glaucoma patients regardless of the response of pressure to treatment.