In a routine cytogenetic investigation of the outpatients of a hospital for the mentally retarded, a 26-year-old women with a presumptive interstitial deletion of the short arm of one of the X chromosomes was found. The same aberration was found in her phenotypically normal mother and in one of her four sisters, all phenotypically normal. By GTG- and QFQ-banding methods, the deletion was interpreted to involve the entire band Xp21 and adjacent parts of p11 and p22. The karyotype is written 46,X,del(X)(pter leads to p22::p11 leads to qter). By autoradiography and Bud R acridine orange technique, the deleted X was the late replicating one in all three affected persons. The deletion apparently causes shortness of stature but no other phenotypic symptoms or signs. Hence a gene or genes controlling stature is located in band Xp21 or regions immediately adjacent to this band. Since the absence of this region does not cause streak gonads, it does not contain genes controlling the formation of the ovaries. This appears to be the first example of a heritable chromosome deletion compatible with a normal phenotype and reproduction.