Serial volumetric intravascular ultrasound (IVUS) was used to study de novo, nontreated left main coronary arteries (LMCAs) in 31 patients. Using an automated contour detection algorithm, analysis of 7.2 ± 2.5 mm long segments included arterial, lumen, and plaque volumes and plaque burden (plaque/arterial volumes). During follow-up (7.7 ± 2.4 months), the percent change in lumen volume correlated with the percent change in arterial volume (r = 0.897, p <0.0001), but not with the percent change in plaque volume (r = 0.066, p = 0.7263). Percent changes in arterial volume correlated with percent changes in plaque + media volume (r = 0.448, p = 0.0115), indicating arterial remodeling. However, there was a spectrum of responses ranging from inadequate remodeling (decrease in lumen volume despite no increase or a decrease in plaque volume: i.e., arterial shrinkage) to overcompensation (an increase in lumen volume despite an increase in plaque volume).Serial volumetric IVUS (1) confirms the existence of both positive and negative remodeling in LMCA, and (2) shows that in moderate LMCA disease, luminal changes resulted primarily from positive versus negative remodeling, not plaque progression and/or regression.