A theoretical midgace ratio was determined from midface lengths and heights in straight lateral radiographs. Using regression analyses, the ratio was tested on 90 adults who did not have an aberrant anatomy of the maxilla. A linear relationship was found between the theoretical midface ratio and sella-nasion-point A (SNA), and the correlation was high (adjustedR2 = 82.7%). Limit values were determined using the theoretical midface ratio for the classifications of retrognathia, orthognathia, and prognathia. Conditional probabilities were estimated for classifications according to SNA for every classification by the theoretical midface ratio. Eighty percent of the subjects were identically classified by SNA and the theoretical midface ratio, and no subjects were placed differently in the extreme groups of retrognathia and prognathia. The theoretical midface ratio was then used to estimate the degree of prognathism in 35 adults with maxillonasal dysplasia. Less than half of the subjects had maxillary retrognathia, and the majority had an orthgnathic or even prognathic maxilla. This is an indication that individualization of treatment objectives, including the consideration of mandibular setback operations to correct incisor crossbites, is equally important in maxillonasal dyspl asia and in normal anatomy.