We report a retrospective study of 60 eyes from 46 patients all with primary open-angle glaucoma who had a trabeculectomy to one or both eyes. A minimum five-year follow-up was available. The percentage pressure reductions at the one and five years postoperatively is shown. In a subgroup of 50 eyes which had Goldman fields documented at least six-monthly for a minimum of five years postoperatively there was a continuation of glaucomatous field loss in 18% of eyes despite postoperative pressures in the normal range. The five-year percentage pressure reduction and the minimum percentage pressure reductions were statistically significant in a comparison between the field loss and no field loss groups. However, in the individual case there seemed no way of predicting which eyes would develop postoperative field loss, and regular postoperative perimetric assessment was found to be essential whatever the pressure measurements recorded.