The mechanisms of immediate and late changes after percutaneous transluminal coronary angioplasty (PTCA) and directional coronary atherectomy (DCA) were assessed by serial ultrasound imaging in 18 patients treated with PTCA and 16 treated with DCA before, immediately after, and 6 months after coronary interventions. A reduction in plaque area was the main operative mechanism of DCA, explaining 66% of lumen enlargement. In the PTCA group, the increase in lumen area was the result of a more balanced combination of plaque reduction (52% of lumen increase) and increase in total lumen area (48%); p < 0.05 versus DCA. In the PTCA group, this last mechanism was prevalent (p < 0.05) in the lesions showing wall fracture or dissection after treatment and in the lesions with a mixed or calcific composition. In the PTCA group, concentric lesions showed a greater plaque compression than eccentric lesions (p < 0.02). Plaque increase was responsible for 92% and 32% of the late lumen loss after DCA and after PTCA, respectively (p < 0.05). In PTCA patients, a chronic reduction in total vessel area was the main operative mechanism of lumen reduction (67%) and was prevalent in lesions with a mixed or calcific composition. (p < 0.05).