A set of descriptors was developed and used to code patients with 4p or 9p monosomy or trisomy, “blind” as to their karyotype. Techniques of numerical taxonomy were used to classify the patients on the basis of their phenotypic resemblance. As expected, the results confirm strong phenotype‐karyotype correlations. When cytogenetic interpretations are uncertain, the phenotypic findings may confirm or refute the interpretations. The approach has practical implications, providing further knowledge of phenotypic effects of specific chromosomal segments, which will aid cytogeneticists in their search for karyotypic defects. More important, the results also serve as an excellent model for developing strategies for the classification of syndromes of unknown eti