Of 119 patients with the Apert syndrome, 94 pedigrees are available for study. Selected family histories and recorded events during pregnancy are also reported. The sex ratio is 1:1 (60 males, 59 females). All 94 pedigrees except one (an affected father and daughter) represent sporadic instances. Combining our cases with those available in the literature, only nine familial instances are known to date. The familial cases, the equal number of affected males and females, and the increased paternal age in sporadic cases strongly suggest autosomal dominant inheritance. The rarity of familial instances can be explained by reduced genetic fitness of affected individuals because severe malformations and the presence of mental deficiency, found in many cases, diminish desirability as mates. The variability of the syndrome has been defined mainly on the basis of sporadic cases. High resolution banding was normal in five recently studied cases.