The chromosome constitution of a 4 year old retarded girl was recently re-evaluated using banding methods. Initial chromosome analysis performed at another institution using the conventional method was interpreted as 45, XX, -G. The patient was the full term 5 Ib 14 oz product of a second pregnancy to a 23 year old mother. There was no history of fetal wastage. The positive physical findings included failure to thrive, moderate psychomotor retardation, mildly dysmorphic facies with a prominent nasal bridge and large ears, low set nipples, systolic ejection murmur, hypoplastic labia majora, generalized spasticity with limited hip abduction, knee extension, and distally located palmar triradii (t”). With the combination of Q, G and R banding methods, it is evident that the patient has an unbalanced insertion type of translocation, 45, XX, -17, -21, +ins (17;21) (17pter→17q23: 21q11→21q22:sl7q23→17qter). The distal portion of 21q22 has been deleted. Apparently the absence of a portion of the euchromatic region of 21q22 is responsible for the patient's abnormal phenotype. The patient shows some features of 21 deletion syndrome. Both parents have normal chromosomes. Since complete autosomal monosomy is extremely rare, even in studies of early spontaneous abortion material, all liveborns with apparent monosomy must be re-evaluated with a combination of banding methods to search for an unbalanced translocation resulting in a partial deletion. If such a structural aberration is not found, multiple tissues should be studied to search for 45/46 mosaicism.