This study analysed 90 patients who underwent a bi-maxillary osteotomy for a class III malocclusion. There were 30 in each of three groups of patients based upon the Frankfort mandibular plane angle (FMA). They were divided into: greater than 32°, 22°-32°, and less than 22° as high, medium and low groups. The surgical outcome and stability were compared, and it was shown that those with a low FMA had a more consistent relapse than those with a normal or high FMA. Relapse was always in a forward and upward manner. Therefore, whilst finding that a bi-maxillary osteotomy is stable for mandibular prognathism regardless of the vertical facial pattern, they advocate that over correction of 1-1½mm in the mandibular setback was worth considering in those with a low FMA angle. They did not assess the cause of the relapse, whether this was due to positional changes, model of the condyle or soft tissue tension.