Mapstone introduced the term 'partial angle closure' to describe how eyes with apparently open angles could be damaged by a covert angle closing mechanism, identifiable by a positive result of the pilocarpine phenylephrine provocative test (PPPT). This paper presents a 10-year follow-up of 68 patients with narrow but open anterior chamber angles who, on the basis of a positive PPPT, underwent peripheral iridectomy (PI) or laser iridotomy LI). In the 42 patients with glaucoma or ocular hypertension the mean intraocular pressure (IOP) was unchanged following iridectomy; 45% of eyes which showed glaucomatous optic disc damage at presentation and 25.6% of ocular hypertensive eyes subsequently required trabeculectomy to achieve IOP control. In 16 patients with no ocular abnormality other than a narrow angle 19 eyes had positive PPPTs and underwent PI. After a 10-year follow-up only one of the 13 untreated fellow eyes developed angle closure glaucoma. As this is usually bilateral, many of the positive tests in this group of patients may therefore have been false positives. This study shows that eyes with narrow angles which had positive PPPT results did not benefit from PI iridectomy or LI and therefore a positive PPPT is not indicative of the presence of an angle closing mechanism.