Pelvic support osteotomies for chronically dislocated hips improve stability but result in limb-length discrepancy and valgus deformity of the knee. We prospectively evaluated and followed up 16 patients (14 females, two males) to ascertain whether a monolateral external fixator could achieve a pelvic support osteotomy and eliminate the limb-length discrepancy without disturbing knee motion. The patients had a mean age of 25.3 years at the time of surgery. The Harris hip score, knee range of motion, and Trendelenburg sign were evaluated preoperatively and at followup. The average time from fixator application until removal was 7 months (range, 5.2-9.5 months). The followup averaged 52.5 months (range, 26-84 months). The mean Harris hip score increased from 50 points (range, 32-73 points) preoperatively to 87.6 points (range, 67-98 points) at last followup. Four patients retained a positive Trendelenburg sign. At the time of fixator removal, 88% of preoperative knee range of motion was retained, and preoperative knee motion was achieved at last followup in all patients. The monolateral external fixator was suitable for a pelvic support osteotomy, equalizing limb-length discrepancy, and obtaining lower extremity alignment. This method achieved a pain-free, functional hip and good knee motion.