Objectives: The aims of this clinical study were to compare internal three-dimensional (3D) adaptation of porcelain laminate veneers (PLV) with minimal tooth preparation and without tooth preparation (prepless) and to evaluate the clinical outcomes at baseline and following 6, 12, and 24 months after luting. Method and Materials: Thirty-one prepless PLV and 31 PLV with minimal tooth preparation were fabricated using lithium disilicate glass-ceramic material and placed in 12 patients (8 women, 4 men; 18 to 40 years old). All PLV were luted with an adhesive luting system (Variolink veneer). A silicone replica was obtained to measure internal adaptation of each PLV using a low viscosity polyvinyl siloxane impression material just before luting. Silicone replicas were scanned in x-ray micro computerized tomography (micro CT). Clinical evaluations took place at baseline (2 days after luting) and following 6, 12, and 24 months after luting. Marginal integrity, marginal discoloration, secondary caries, tooth sensitivity, and fracture were evaluated following FDI criteria. Replica scores were analyzed using Mann-Whitney U and Student's t test (a = .05). Kaplan-Meier statistical analysis was done for the survival rate of PLV. FDI criteria scores were analyzed using Pearson's chi-square test (a = .05). Results: The median marginal gaps for PLV-without-tooth-preparation and PLV-with-minimal-tooth-preparation groups were 100 mu m and 140 mu m respectively. There was a statistically significant difference between the two groups with respect to marginal gap (P = .04). The mean internal adaptation for the PLV-without-tooth-preparation group was 217.17 +/- 54.72 mu m, and was 170.67 +/- 46.54 mu m for the PLV-with-minimal-tooth-preparation group. There was a statistically significant difference between the two groups (P = .001). Based on FDI criteria, 100% of the PLV were rated satisfactory during the 2-year period. Conclusions: In this in-vivo study, mean and median values of marginal gap and internal adaptation for PLV with minimal tooth preparation and PLVs without tooth preparation were within a clinically acceptable range. A 100% success rate was recorded for all PLV during the 2-year period.