Objective: The purpose of this retrospective study was to determine the efficacy of Invisalign in a large sample of patients compared to fixed appliances. Method and Materials: The test group consisted of 100 patients treated with Invisalign compared with a control group treated with conventional fixed appliances matched for sex, age, and initial severity of malocclusion based on the amount of anterior dental crowding (Little Index) and the Peer Assessment Rating (PAR Index) scores. The retainer used was a 0.0175-inch multistranded stainless-steel wire bonded from canine-to-canine in the mandibular arch and from lateral inisor-to-lateral incisor in the maxillary arch. A paired t test was used to compare both initial and final PAR scores. Results: There was an overall 80.9% improvement, and 63 subjects did not need any refinement. The mean number of aligners used was 14 (+ 15 for the refinements) in the maxillary arch and 29 (+ 14 for the refinements) in the mandibular arch. The mean duration of treatment was 14 months (+ 7 months for the refinements). Significant statistical differences were found in the posttreatment scores, within both the Invisalign group and the control group. No differences were found in the follow-up scores. Additionally, the duration of treatment was 4 months longer in the control group. Conclusion: More than 90% of the subjects treated with Invisalign achieved a significant improvement, as shown by the PAR scores. A need for additional aligners was reported for 37% of the patients. Fixed bonded retainers seem to be a good option in preventing tooth relapse after Invisalign and fixed conventional treatments. Keywords: aligner, Invisalign, occlusion, orthodontic appliances, orthodontic treatment, treatment outcome