The effect of cataract surgery in patients with glaucoma controlled by either topical medication or surgery was assessed in 64 patients. At one year there was a small significant fall in intraocular pressure (IOP) for eyes without previous surgery (preoperative IOP 18.9 +/- 4.7, range 12 to 35 mmHg; postoperative IOP 16.3 +/- 3.4, range 10 to 26 mmHg; P < 0.01) and also for those with previous surgery (preoperative IOP 15.0 +/- 4.3, range 3 to 22 mmHg; postoperative IOP 14.2 +/- 3.7, range 6 to 22 mmHg, P < 0.05). There was a significantly greater incidence of high rise in IOP to 30 mmHg or more immediately after operation in patients without (32%) than those with previous surgery (13%) (chi 2 = 3.9; P < 0.05). Complications were minimal in each group. Iridotomy to deliver the nucleus was necessary in nine eyes without and 21 with previous surgery. Cataract extraction usually causes only a small fall in IOP in glaucomatous patients. If a separate corneal section is used there is no loss of function of the filtering bleb in patients with previous glaucoma surgery.