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AIM:To observe related biological parameters of 3 minutes dark-room provocative test in patients with laser peripheral iridectomy(LPI)in the fellow eyes of acute primary angle-closure(APAC)by ultrasound biomicroscopy(UBM). To explore the risk factors in primary angle closure suspect(PACS)patients with progressive angle closure after LPI. <p>METHODS: Seventy-eight eyes of APAC patients without peripheral anterior synechia were selected. Each eye underwent 3 minutes dark-room provocative test after LPI. Anterior segment parameters, including anterior chamber depth(ACD), anterior chamber angle open distance500(AOD₅₀₀), peripheral iris thickness(PIT), iris convex(IC), the position of iris insertion and trabecular-ciliary process distance(TCPD), and the number of positional angle closure(NPAC)were observed and analyzed by statistic methods. <p>RESULTS:Patients with APAC were examined by UBM after LPI and 26 eyes(33%)occurs at least one positional angle closure,19 eyes(24%)were positive in 3 minutes dark-room provocative test among them. It occurs a positive relationship between the elevation intraocular pressure and the number of positional angle closure in dark-room provocative test(<i>r</i>=0.84, <i>P</i><0.01). AOD₅₀₀, IT and IC were significantly changed from normal light to darkroom between positional angle closure positive group and positional angle closure negative group(all <i>P</i><0.01). In single factor analysis, AOD₅₀₀(<i>P</i>=0.003), IT(<i>P</i>=0.012), IC(<i>P</i>=0.043), TPCD(<i>P</i>=0.015), the position of iris insertion(<i>P</i>=0.024)were correlative factors of positive results. In multiple-factor analysis, only IT(<i>P</i>=0.011), TPCD(<i>P</i>=0.009), iris root attachment points(<i>P</i>=0.02)were independent risk factors of positive results. <p>CONCLUSION:A certain proportion of patients with PACS after LPI appeared positional angle closure in a dark room. Peripheral iris hypertrophy, anterior displacement of the ciliary body and iris root attachment points are vital risk factors. Long-term follow-up study and intervention treatment are required in these patients after LPI.