Clinical and cytogenetic studies are reported on 13 patients with Wolf-Hirschhorn syndrome. The oldest of the living twelve probands is 24 years of age. Three of these patients had a translocation involving the short arm of chromosome 4, and in one of these the anomalous chromosome was inherited from the father. Another three patients were believed, on the basis of GTG-staining, to have a translocation although the origin of the translocated chromatin could not be identified. In the remaining seven patients the anomalous chromosome appeared to be a simple deletion, although in two cases a translocation could not be ruled out. Cytogenetic studies in these patients suggest that the critical deletion involved in Wolf-Hirschhorn syndrome is within 4p16.