Nine new cases of histidinemia have been found in four different sibships and in two successive generations in the same family. No history of consanguinity is present. The father of the propositus is normal. The pattern of inheritance appears to be autosomal dominant. Since these patients have absent or decreased skin histidase activity and they excrete decreased amounts of N-formiminoglutamic acid following histidine loading, they probably represent an intermediate genotype between the typical histidinemics and individuals with normal skin histidase.