Chromosomal syndromes may result from extremely small cytogenetic alterations, involving as little as one chromosomal sub-band. An example is the cri-du-chat (cat cry) syndrome, in which the critical deletion appears to involve the sub-bands 5p15.1-3. Aside from a sporadic deletion of 5p, the loss of material may result from an interstitial deletion, caused by the malsegregation of a balanced parental translocation, or, in exceptional cases, as the consequence of a sporadic or familial chromosomal inversion which has been modified by unequal crossing-over (recombination aneuploidy). In addition, in certain children with the clinical syndrome (11 of 331 in a recent review) the deletion cannot be proven cytogenetically and is presumed to be submicroscopic. In the case described here, an intrachromosomal rearrangement of chromosome 5--an invper(5)(p15q14 or 15) or an ins(5) (p15q12q12)--is segregating in the maternal family. Three of the four children born to the couple were abnormal. The first boy, affected with cleft lip, pyloric stenosis and inguinal hernias, died at 4 months of age. The second died at 3 weeks with microcephaly and agenesis of the corpus callosum, cleft palate, heart malformation, and sexual ambiguity. A third boy, now 14 years old, is phenotypically normal and has a normal karyotype. The female proband, seen by us at 9 years of age, showed the clinical features of the cri-du-chat syndrome, with severe psychomotor and staturoponderal retardation, facial dysmorphism, congenital heart defect, and the peculiar voice for which she had received the nickname of "kitten". Her karyotype shows the same variation of chromosome 5 present in her mother and grandmother, characterized on G bands by an additional dark band on 5p15. As there is no evidence for a reciprocal translocation in the mother, the most probable explanation is that of a familial inversion or insertion within chromosome 5. This rearrangement, subject to meiotic modifications, could have been responsible for the complex malformations observed in 3 of the 4 children, including our proband who has a clinical diagnosis of the cri-du-chat syndrome.