Percutaneous angioscopy during coronary angioplasty using a steerable microangioscope. The feasibility of using a flexible, steerable angioscope to perform coronary angioscopy before and after percutaneous coronary angioplasty was tested. The microangioscope fits through an 8F coronary angioplasty guiding catheter and contains a multifiber viewing bundle incorporated into the body of a 4.3F balloon catheter with a central lumen for distal flushing and guide-wire passage. Angioscopy was performed without complications 45 times in 24 patients, including 6 patients with stable and 18 with unstable angina. Circumferential visualization of the target lesion was successful in 20 (83%) of the 24 patients and improved with operator experience. Excellent visualization of the target lesion was achieved in 16 (94%) of the last 17 patients. Plaque, thrombus and dissection were among the abnormal findings in the 20 patients (4 with stable, 16 with unstable angina) in whom circumferential viewing of the target lesion was achieved. In four patients with restenosis after angioplasty, the lesion morphology was distinctly different from that of lesions in arteries without prior angioplasty. In patients with stable angina, no thrombus or dissection was seen by angiography or angioscopy before angioplasty. In patients with unstable angina, thrombus was detected more frequently by angioscopy than by angiography before angioplasty (8 versus 2 of 16) and after (15 versus 2 of 16) angioplasty. Intimal dissection was also seen much more frequently by angioscopy than by angiography before angioplasty (7 versus 0 of 16) and after angioplasty (16 versus 7 of 16). It is concluded that high resolution percutaneous coronary angioscopy can be performed safely in conjunction with balloon angioplasty. Further investigation is needed before this diagnostic tool can be applied clinically. 