There are few in vivo data concerning the mechanisms of balloon inflation during coronary angioplasty. To characterize how lesions dilate, videodensitometry was used to measure the diameter of the inflated balloon across 29 coronary lesions in 27 patients. Pressure-diameter curves for each lesion were derived with use of a standardized incremental inflation protocol in which pressures between 2 and 6 atm in 3 mm low profile balloons approximated normal vessel diameter. The diameter of coronary stenosis before and after angioplasty was also measured. Pressure-diameter curves showed that the most improvement in luminal caliber occurred at low inflation pressure. A distensibility factor was defined as the ratio of the amount of balloon inflation at 2 atm compared with the balloon diameter at 6 atm. Eccentric irregular lesions (n = 11) had a greater distensibility factor (0.49 +/- 0.17) than did lesions (n = 18) without this configuration (0.33 +/- 0.14) (p less than 0.02). The former were soft, presumably because of thrombus in these lesions. In addition, there were no differences in patterns of balloon inflation for lesions requiring additional inflation or for dilations resulting in an intimal crack or dissection after angioplasty. There was often a loss of luminal caliber when balloon diameter at 6 atm was compared with the diameter after angioplasty. This was defined as elasticity or recoil. There was a significant direct correlation between the amount of elasticity and the extent of balloon inflation at 6 atm (that is, lesions more fully dilated at 6 atm showed more elasticity).(ABSTRACT TRUNCATED AT 250 WORDS)