Introduction and Purposes. Custom made acetabular prosthesis are a valid option for the reconstruction after the resection of pelvic tumors. They should guarantee a stable and reliable reconstruction for the expected survival of the patient. Nevertheless in many cases periacetabular metastatic lesions have been compared to high grade (IIIA-B) Paprosky defects, but treated with low or intermediate longevity implants. Some complex post-traumatic scenarios or total hip arthroplasty (THA) multiple failures need a reconstruction according to oncologic criteria to fill in the huge defect and to obtain an acceptable function. The aim of the study is to compare 3D custom-made implants for tumors and for THA failures. Materials and Methods. Three custom-made implants after tumor resection (group A: 1 chondroblastic osteosarcoma, 1 bifasic synovialsarcoma, 1 high grade chondrosarcoma) were evaluated and compared to 3 acetabular complex reconstructions after non-oncologic bone defect (group B: 3 cases of aseptic loosening after at least 2 revisions). All the implants were case-based designed, 3D printed, and realized with porous or trabecular surfaces on a Titanium base prosthesis. Age range 16–70 ys in oncologic patients and 60–75 ys in non-oncologic patients. The bone defect to be reconstructed after tumor resection was classified according to Enneking zones (1 type 1-2-3 resection, 1 type 2 resection, 1 partial type 2 resection). Non-oncologic cases were comparable in term of remaining bone stock and classified according to Paprosky classification for acetabular defects as 1 type IIIA an 2 type IIIB. Complications, MSTS functional score, necessity of walking-aids were evaluated at minimum follow up of 1 year. Results. In both groups, good functional results were obtained (MSTS score 25/30 in both groups). No cases of aseptic loosening and no infection occured. After 3 months of partial or no weight-bearing on the operated limb, 3 patients were able to walk unaided and 3 walked with one cane or crutch. No limb length discrepancy (major than 2 cm) were observed. Limping was present in oncologic cases due to muscle resection. Overall better results in term of satisfaction and quality of life were obtained in younger (oncologic) patients. Conclusions. Complex THA revision cases can sometimes be considered for a reconstruction with oncologic criteria. Even if indications are limited an acceptable functional results can be obtained. In the past for these cases a Girldestone resection was the only option. A collaboration among orthopaedic oncologist and hip arthroplasty surgeons is advisable in major orthopaedic centers to improve the prosthetic design and the functional results