This study of chromosomal syndromes was based on phenotypic data for two hundred and sixty-three individuals ascertained from case reports in the literature supplemented by unpublished reports. The individuals used in the study had at least one chromosomal duplication or deletion of a segment of chromosome 3, excluding a group of fifteen individuals with an unknown phenotypic etiology (Cornelia de Lange), which was used as a control group. Numerical taxonomy techniques were carried out on a data set based on one hundred and twelve structural phenotypic variables to generate and identify clusters containing individuals with like phenotypes. Th results of the classification were then compared with the karyotypes of the individuals in each phenotypic group to identify a chromosomal basis for like phenotypes, as well as to identify components of a phenotype due to a recombinant chromosome 3 with a duplication of 3q and a deletion of 3p. Four separate cluster analyses were carried out in this study. (Abstract shortened by UMI.)