Objectives: Femoroacetabular impingement (FAI) likely represents the most common mechanism that leads to early cartilage and labral damage in the nondysplastic hip. Recently, hip arthroscopy has been used as a surgical approach to address both labral pathology and osteochondral deformities. Incomplete correction of femoral offset and sphericity is the leading cause for revision surgery for symptomatic FAI. Due to the technical difficulty of arthroscopic assessment of the hip, pre-operative planning and understanding of the underlying deformity is critical for successful treatment. The purpose of this study is to assess the utility of a novel CT-based three-dimensional software program for preoperative planning for arthroscopic treatment of femoroacetabular impingement. Our hypothesis is that preoperative planning using a 3-D CT software program results in improved accuracy of correction and will reduce the incidence of under-resection. Methods: Arthroscopic femoral neck osteoplasty and acetabular rim resection with labral repair or refixation was performed in all patients with symptomatic FAI that have failed nonsurgical management. Patients were distributed into two treatment groups: 36 patients in the Plan group utilized preoperative planning using a 3-D CT-based, computer software program (Dyonics Plan®, Smith and Nephew); and 30 patients in the control group utilized our standard preoperative assessment of plain radiographs and a CT scan with 3-D reconstructions. Pre- and postoperative AP alpha angle, extended neck lateral (Dunn) alpha angle, anterior head-neck offset, and beta angle were compared between both groups to assess the degree of deformity correction. Under-resection was defined as a post-operative alpha angle > 50° or a reduction < 20° for cases of very large alpha angles. Results: Mixed model analysis of variance was used to compare radiographic indices between the control and Plan groups. There were no significant differences between the two groups at baseline. Both groups had a statistically significant improvement in all radiographic measurements after surgery. The Dunn alpha angle improved an average of 17.9° in the Plan group as compared to an average of 12.6° in the control group (p=0.003). The anterior head-neck offset improved a mean of 4.8 mm in the Plan group as compared to 3.5 mm in the control group (p=0.007). The number of patients with radiographic evidence of under-resection improved from 26.7% in the control group to 2.8% in the Plan group (p=0.009). The maximum alpha angles obtained from pre-operative CT scans were an average of 3° greater than the maximum alpha angles on the AP and Dunn X-ray views (p=0.007) Conclusion: Arthroscopic osteoplasty utilizing the preoperative planning software resulted in a significant decrease in the rate of inadequate resection and an improvement in accuracy of deformity correction. Additionally, radial imaging with CT scan and the planning software measured a significantly greater maximum alpha angle than AP and Dunn view X-rays. With an improved accuracy of resection, preoperative planning using a 3-D CT software program may help to minimize the frequency of under-resection and thereby decrease the incidence of revision surgery.