To determine the effect of apraclonidine hydrochloride on the acute intraocular pressure (IOP) rise after argon laser iridotomy (ALI), a double-masked comparative study was carried out. Twenty-nine eyes (20 patients) with angle-closure glaucoma underwent ALI. Eighteen eyes were treated with apraclonidine, and the remainder received a placebo 1 hour before and immediately after ALI. The mean IOP increase in the apraclonidine group was lower than that in the placebo group at each postlaser interval (p < 0.01). Although the average value of the maximal increases of IOP after ALI in the apraclonidine group was 4 mmHg, that in the placebo group was 16 mmHg. In the placebo group, 27.3% (3 out of 11 eyes) experienced an IOP rise > or = 10 mmHg. However, that kind of IOP rise was not found in the apraclonidine group (0 out of 18 eyes) (p < 0.01). Ocular or systemic side effects were not found in a series of examinations in both groups. Therefore, apraclonidine proved to be effective in lowering the IOP rise after ALI.