The treatment of a missed Monteggia (MM) fracture dislocation is still controversial. We describe our initial experience with ulnar osteotomy and progressive correction with unilateral external fixator in MM. We retrospectively evaluated 20 children undergoing ulnar osteotomy and progressive distraction angulation by unilateral external fixator to treat MM. Nine patients had closed reduction, whereas 11 patients had simultaneous open reduction, repair, or reconstruction of the annular ligament and K-wire stabilization of the radiocapitellar joint. Patients were followed for an average of 3 (1-11) years. Three children developed aseptic nonunion and one child had delayed union. A distal level of the osteotomy significantly increased the rate of nonunion or delayed union. At the final follow-up, eight children had complete reduction of the radial head, six children had partial reduction, whereas in six cases, the radial head remained dislocated. The angulation and the level of the osteotomy significantly influenced the relocation, whereas the open reduction had no significant effect on the final position of the radial head. At the final follow-up, the Kim's score averaged 93.25. The flexion-extension arc significantly improved postoperatively, and it was positively correlated with the angulation. The ulnar osteotomy and progressive traction-angulation by unilateral external fixator can achieve satisfactory results in MM, if a meticulous surgical technique is applied; care must be taken regarding the level of osteotomy and the progressive traction-angulation.