Opening wedge high tibial osteotomy (HTO) for varus knee osteoarthritis has shown several advantages over the classic closing wedge technique. The aim of the current prospective study was to evaluate mid-term clinical and radiographic results, as well as complications, of medial opening wedge osteotomy using the hemicallotasis technique. Forty-nine high tibial oste-otomies were performed for unilateral varus primary osteoarthritis from 1999 to 2002. A medial incomplete osteotomy was performed after elevating the superficial collateral ligament. Four pins were inserted, two hydroxyapatite-coated in the metaphyseal bone, and two standard conical pins in the diaphyseal bone. The correction started 4–5 days postoperatively. The patient managed the correction by adjusting half of a turn twice each day. When the desired correction was achieved, the device was locked. Eight-to-nine weeks after surgery, the radiographic healing was evaluated, and if adequate, the device was removed on a outpatient basis. The mean age of the patients was 57 years (range, 32–70 years). The mean follow-up was 5 years (range, 4–7 years). The mean hip-knee-ankle angle (HKA) was 167 (range 164–171) deg preoperatively and 182 (range, 176–186) deg at follow up. We did not observe any early or late collapse of the new bone wedge. Union was achieved in all patients, and the mean time to fixation was 69 (range 60–85) days. Knee Society score improved from 52 points preoperatively to 93 at follow up. Eighty-five percent of the patients showed excellent-to-good clinical outcome. None of the knees had required revision surgery at follow-up. No meta-diaphyseal mismatch was noted on both the sagittal and coronal plain at radiographic analysis. Patellar height (IS ratio) reduced, on average, from 1.1 (±0.4) to 0.9 (±0.4), but no patella was found to be baja. Complications included a number of superficial infection uneventfully healed such as two cellulitis with erysipelas-like rushes, and five minor (grade I-II) pin tract infections. There were two technical problems. One obese patient developed an undisplaced inter-condylar fracture of the proximal tibial osteotomized fragment, which subsequently healed and the patient achieved a good clinical outcome. In another patient the anterior pin on the metaphyseal fragment was positioned too anteriorly, and was thereafter repositioned. There were no neurologic or vascular complications. Using the hemicallotasis technique for HTO the authors obtained a precise correction with a relatively low complication rate. The use of an external fixator allowed quick surgery, early weight-bearing, immediate knee motion, avoiding permanent hardware on bone. Conversion to a total knee arthroplasty seems to be easy when this technique has been employed, but the influence of pin tract infection on possible septic failures remains to be determined.