A population of 80 home-reared children with cri du chat syndrome was investigated to document the clinical heterogeneity of the syndrome and to analyze the factors influencing the severity of the phenotypic characteristics. When individuals with isolated deletions were compared with those possessing unbalanced translocations involving other chromosomes in addition to number 5, the latter group had a greater incidence of physical anomalies, more frequent hospitalizations, and a higher mortality. Chronic complaints in both groups included upper respiratory tract infection, otitis media, and a previously unrecognized association with gastrointestinal tract anomalies. In children with terminal deletions, there was a significant negative correlation between the size of the deletion and the individual's intelligence quotient. In addition, patients with larger deletions had more severe growth retardation, particularly with respect to the degree of microcephaly. The gradual progression with age of the characteristic facial features remained consistent regardless of differing racial backgrounds and the size of the deletion. Our findings delineate the variation in the clinical and karyotypic features of this syndrome.