Purpose To determine the relationship between appositional angle closure (AppAC) and the conversion of primary angle closure (PAC) into primary angle closure glaucoma (PACG) after laser peripheral iridotomy (LPI). Methods Consecutive patients with PAC were included in this prospective cohort study. The eyes of the included patients who had undergone LPI with five or more reliable visual field (VF) tests in at least 5 years were analysed. Baseline AppAC was diagnosed when the trabecular meshwork and iris were located appositionally on the ultrasound biomicroscopy image. The criteria for PAC conversion were based on development of glaucomatous optic neuropathy and corresponding VF defects. Other factors such as age and vertical cup-to-disc ratio (VCDR) were also investigated by univariate and multivariate logistic regression analyses. Results 128 patients were assessed at baseline. 80 eyes of 58 patients fulfilled the inclusion criteria. The mean follow-up period was 6.67 +/- 1.33 years. PAC conversion was observed in 20 eyes. The frequency of PAC conversion was significantly higher in eyes with AppAC > 2 quadrants at baseline (58.3%, 7/12) than in those with AppAC <= 2 quadrants (19.1%, 13/68; p < 0.05). Compared with the non-converting group, the age and VCDR at baseline were both higher in the converting group (p < 0.05). Wider AppAC, older age and higher VCDR were significantly associated with PAC conversion. Conclusions 25% of PAC eyes of Chinese patients converted into PACG eyes in more than 5 years after LPI. AppAC > 2 quadrants were related to the PAC conversion. Older baseline age and higher VCDR were also associated with PAC conversion.