In this study the influence of plaque composition on mechanism of immediate lumen enlargement after PTCA were assessed by serial ultrasound imaging in 77 patients. According to the preprocedural ultrasonic plaque characteristics, the lesions were classified as soft (54.5%), diffusely calcified (14.3%), and mixed (31.2%). The total arc of calcium and plaque eccentricity were also calculated. The mean balloon/artery ratio was equal for all types of lesions (1.07 ± 0.19), but diffusely calcified lesions required a higher maximal balloon inflation pressure (11.3 ± 3.75 atm vs 9.0 ± 3.15 atm for calcified and soft lesions, respectively, p < 0.05). The increase in lumen area after PTCA was similar in soft and mixed lesions (3.49 ± 1.61 mm2 vs 3.56 ± 1.58 mm2, NS) and was slightly lower in diffusely calcified lesions (2.97 ± 1.33 mm2, NS). Reduction in plaque area was the main operative mechanism in lesions with soft plaque, explaining 77.9% of the acute lumen gain. Expansion of the total vessel area was the predominant mechanism of lumen area increase in lesions with calcified and mixed plaques (99.1% vs 74.2%, respectively, NS). In particular, lesions with a total arc of calcium >90 degrees showed a high prevalence of vessel expansion as a mechanism of lumen enlargement compared with lesions with a calcium arc <90° (79.3% vs 23.8%, respectively, p < 0.05). In the studied group, presence of calcification was highly associated with wall fracture and dissection after balloon dilatation (93.4% vs 6.6% in lesions with and without calcifications, respectively, p < 0.05). A trend toward a greater degree of plaque reduction in concentric lesions in comparison to eccentric lesions was observed (58.3% vs 48.5%, respectively, NS). In conclusion, immediate lumen enlargement after PTCA does not differ for soft and mixed plaques. Diffusely calcific plaques show a smaller lumen enlargement despite a higher balloon inflation pressure. Plaque compression or axial redistribution is the main operative mechanism of PTCA in soft plaques, whereas an increase in total vessel area determines the lumen enlargement in mixed/calcific plaques. Plaque fracture or wall dissection is almost invariably associated with presence of plaque calcification.