This is a prospective observational study of 342 subjects over four years. Sixty-five were controls who had open angles both on gonioscopy and AS-OCT, while 277 had open angles only on gonioscopy but iridotrabecular contact (ITC) on AS-OCT (one to four quadrants). At the end of the four year study period, 27.4% of those with ITC on AS-OCT at baseline developed gonioscopic angle closure in two or more quadrants, in contrast to none in the control subjects. The greater the amount of ITC on AS-OCT, the higher the likelihood of gonioscopic angle closure occurring after four years. This study confirms what many angle closure glaucoma specialists have long suspected – AS-OCT detects angle closure earlier than gonioscopy and will play an increasingly important role in the management of angle closure suspect patients.