Data for 331 cri du chat cases, including 34 Danish probands, are reviewed. The incidence nad the prevalence among the mentally retarded population amounted to 1/45,000 and 1.5/1000, respectively. No striking association with prenatal events, parental ages, or birth order could be demonstrated. There was a significant excess of females. Parental translocations were present in slightly more than 10% of the families, while more rare cytogenetic aberrations (mosaicism, rings, and de novo translocations) accounted for less than 10% of all cases. The phenotypically relevant segment has been narrowed down to the midportion of the 5p15 band. Clinical, radiologic, and dermatoglyphic features are summarized and discussed, with special attention to the abnormal cry, which persists in many older probands, and to developmental abnormalities. No obvious correlation could be detected between clinical features and the localization of the deletion. No marker locus has yet been assigned to the short arm of chromosome 5. Treatment and prevention are briefly discussed.