High-resolution chromosome banding and in situ hybridization with combined cosmid and alphoid sequence probes were used to delineate a very small reciprocal translocation in a mother and her two children. The first child has a 46,XX,der(4)t(4;5)(p16.3;p15.3)mat and thus has a deletion of 4p16.3-->pter and a duplication of 5p15.3-->pter (most likely 5p15.31-->pter). Clinical findings include marked growth retardation, developmental delay, seizure disorder, microcephaly, unruly hair, broad nasal tip, downturned mouth, narrow palate, 11 pairs of ribs, mild right club foot, and a deep sacral dimple. Thus, this child has only a few non-specific manifestations of Wolf-Hirschhorn syndrome. The second child has a 46,XY,der(5)t(4;5)(p16.3;p15.3)mat; thus a deletion of 5p15.3-->pter and a duplication of 4p16.3-->pter. He has failure to thrive, developmental delay, microcephaly, sparse hair, horizontal nystagmus, short upturned nose with flared nostrils, thin lips with overhanging upper lip, long fingers and toes, and hypertonicity. Findings in the second patient are not suggestive of cri du chat syndrome (del 5p). The mother is phenotypically normal. This translocation will be useful in mapping genes and markers on the 4p and 5p chromosomal regions.