To examine the balloon inflation pressures required for successful percutaneous transluminal coronary angioplasty (PTCA), the maximal inflation pressure required for 477 coronary lesions in 200 consecutive patients was determined retrospectively. When graded balloon inflations just sufficient to achieve full expansion were used, the maximal inflation pressure used was less than or equal to 8 atm in 412 stenoses (86%) and was less than or equal to 10 atm in 463 stenoses (97%). Successful PTCA was achieved in 98% of lesions with a 3.5% major procedural complication rate. In a second group of 100 patients studied prospectively, the inflation pressure required to achieve full balloon expansion was less than or equal to 8 atm in 214 of 232 stenoses (92%) and less than or equal to 10 atm in 228 stenoses (98%). Thus, PTCA of coronary stenoses can be achieved with high success rates and low complication rates when graded inflations to pressures just sufficient to achieve full balloon expansion are performed. Most coronary stenoses will respond to pressures less than or equal to 8-10 atm.