Background Primary angle-closure glaucoma (PACG) is a major cause of visual morbidity in East Asia. Dark-room provocative test (DRPT) has been used to determine which narrow angles have the risk to develop angle closure. However, the accuracy of DRPT might be altered because that after emerging from the dark room, the configuration of the angle is affected by the light of the slit-lamp and the appositionally closed angle reopens. The aim of this study was to examine the pupillary diameter in different light conditions and use it as a parameter to assess the accuracy of dark-room provocative test. Methods Patients with suspected primary angle-closure glaucoma undergoing DRPT were recruited. The anterior chamber angle was examined by anterior segment optical coherence tomography under the following conditions: (1) in standard room illumination; (2) after short-term dark-adaptation and (3) after DRPT Mean values of pupil size and numbers of appositionally closed angle under different conditions were compared. Results A total of 47 eyes of 47 patients were analyzed. The pupil size after DRPT was smaller than that after short-term dark-adaptation (P<0.001) and smaller than that in standard room illumination (P=0.026). The numbers of appositionally closed angles after short-term dark-adaptation were significantly larger than those after DRPT (P<0.001). There was no significant difference between the numbers of appositionally closed angles in standard room illumination and after DRPT (P=0.157). Conclusions Constriction of pupil size immediately after prolonged dark room provocative test may lead to change in the angle configuration, which may lead to false negative results. We suggest a modified protocol of recording intraocular pressure immediately after DRPT and performing gonioscopy following short-term dark adaptation to improve the accuracy of angle closure assessment. Chin Med J 2009;122(21):2620-2623