Background: The proximal oblique sliding closing wedge osteotomy (POSCOW) technique was developed to address moderate to severe hallux valgus deformity. We present a retrospective multicenter study to analyze the midterm radiological and clinical outcomes of patients treated with this type of proximal osteotomy fixed with plates. Materials and Methods: One hundred and forty-four patients (187 feet) were operated on between May 2005 and June 2010 in 2 separate centers. Inclusion criteria were symptomatic moderate to severe incongruent hallux valgus deformity, no significant restriction in the first metatarsophalangeal joint movement, none to minimal degenerative changes in the first metatarsophalangeal or the tarsometatarsal joints, and no hypermobility. The median age was 60 years. The preoperative hallux valgus angle (HV) was 35.6 degrees, intermetatarsal angle (IM) was 15.3 degrees, AOFAS score was 53 points. The median follow-up was 35 months (range, 12-73). A POSCOW osteotomy was performed in all patients and fixed with plates. We recorded the satisfaction rate, postoperative clinical and radiological results, and complications. Results: The patient satisfaction rate was 87%. The mean postoperative HV angle was 12.3 degrees, IM angle 4.8 degrees, AOFAS score 89 points. The mean decrease in the first metatarsal length was 2.2 mm (range, 0-8). Twelve feet (6.4%) with recurrence of the deformity required revision surgeries. Removal of complete or partial hardware was needed in 23 feet (12.3%) for symptomatic hardware. Five feet (2.6%) developed hallux varus but only 2 required surgery. Transfer metatarsalgia was noted in 9 feet (4.8%). Conclusions: The POSCOW osteotomy was an effective and reliable method for relieving pain and improving function. A learning curve was present, as most of the complications happened in the initial cases. To our knowledge, this is the largest reported series of proximal closing wedge osteotomy for hallux valgus deformities. Level of Evidence: Level IV, case series.