A 3 1/2-year-old boy was referred for chromosomal evaluation because of mental and developmental retardation, peculiar facies, and abnormalities of the extremities. Karyotype analysis disclosed the presence of 46 and 47 chromosome cell lines. The 46 chromosome line contained 4 normal G group chromosomes and an abnormally small Y identified by G banding. Further investigation with Q and C band techniques revealed that the missing segment of the Y, the distal long arm, had been translocated to the end of the long arm of a number 6 chromosome. This de novo rearrangement appeared to be balanced and was found in all cells examined. The 47 chromosome line, which had a frequency of 10% in the patient's leucocytes, was identical to the 46 line except for the presence of an additional copy of the small chromosome. The morphology and banding patterns of the two small acrocentrics in the aneuploid line were found to correspond to those of the der (derivative) Y in the euploid line. The cytogenetic findings suggest that the translocation was followed by non-disjunction of one of its products resulting in mosaicism. Possible causes for the clinical and karyotypic abnormalities are discussed.