Twelve adults with maxillary width discrepancy of greater than 5 mm were trated by surgicaly assisted rapid maxillary expansion. The procedure consisted of bilateral zygomatic buttress and midpalatal osteotomies combined with the use of a tooth-borne orthopedic device postoperatively. Mean palatal expansion of 7.5 mm (range of 6 to 13 mm), measured in the first molar region was achieved within 3 weeks in all patients. Expansion remained stable during the 12-month study period, with a mean relapse for the entire group of 0.88 +- 0.48 mm. Morbidity was limited to mild postoperative discomfort. The results of this preliminary study indicated that surgically assisted rapid maxillary expansion is a safe, simple, and relaible procedure for achieving a permanent increase in skeletal maxillary width in adults. Further study is necessary to document the three-dimension movements of the maxillary segments and long-term stability of the skeletal and dental changes.