Twenty-four eyes of 20 patients with chronic open angle glaucoma who had a trabeculectomy were studied retrospectively. Trabeculectomy resulted in a significant lowering of mean intraocular pressures and improvement of pressure control for the group. Progression of visual field defects occurred in all 24 eyes preoperatively, while postoperatively, the fields of 14 eyes showed no change, but the other ten showed further progression. A comparison of the pressures of the eyes showing continued progression after surgery with the eyes showing no further progression revealed no significant difference in the mean preoperative or postoperative pressure or the mean pressure reduction after surgery. Measurements of the quality of the control, however, showed that the quality of pressure control was significantly better in the eyes that did not show progression of field loss postoperatively. Careful postoperative follow-up observation of the visual field remains necessary even after successful surgical pressure reduction.