Dark room tests and prone-position tests in a lighted room and in a dark room were carried out on 64 eyes in 32 normal patients and 37 eyes of 30 patients that had undergone complicated iridectomy. Among the iridectomized eyes, the dark room tests and the prone-position test in lighted room caused elevation of intraocular pressure of more than 8 mm Hg in only 2.7% and 16.2%, respectively. Nearly 60% of the eyes showed positive results when the prone-position test was performed in a dark room. Our findings may provide experimental support for the theory of direct lens block as an angle-closure mechanism.