The authors reviewed 70 eyes of 59 patients who underwent trabeculectomy. We divided the eyes into the flat anterior chamber group (23 eyes) and formed anterior chamber group (47 eyes, control group). The influence of clinical variables on the development of flat anterior chamber, and the effect of flat anterior chamber on the success of trabeculectomy were evaluated. The two groups were similar in terms of many variables, but the preoperative intraocular pressure (27.1 +/- 15.4 vs. 18.1 +/- 8.7 mmHg), incidence of cataract progression (9/18 vs. 10/44) and choroidal detachment (5/23 vs. 2/47) were higher in the flat anterior chamber group (p < 0.05). The overall success rates for the follow-up period of 30 months were similar between the two groups (p > 0.05). However, the short-term (2 to 8 months) postoperative success rates were lower in the flat anterior chamber group (p < 0.05). The development of flat anterior chamber following trabeculectomy can be reduced by lowering the preoperative intraocular pressure, and flat anterior chamber may be harmful to the postoperative course of trabeculectomy.