Percutaneous transluminal coronary angioplasty was performed in 25 patients with unstable angina and in a similar group of 25 patients with stable angina. The frequency of single, double, and triple vessel disease was identical in each group. Technical success was achieved in 22 (81%) out of 27 attempts in those with unstable angina and in 14 (52%) out of 27 attempts in those with stable angina. Vessel occlusion occurred in nine patients, necessitating emergency bypass surgery in four. There was evidence of myocardial infarction in three patients in each group and one patient in the unstable group subsequently died. Twenty eight of 32 successfully treated patients were followed up by means of repeat coronary arteriography, exercise electrocardiography, and clinical assessment after a mean (SD) interval of 14 (7) months. There was angiographic evidence of restenosis in 32% (seven of 22) of lesions in the unstable group and 44% (four of nine) of lesions in the stable group. There were no late infarctions or deaths during the follow up period. These results support the growing evidence that angioplasty can be carried out safely and effectively in patients with unstable angina.