A low-birth-weight near-term male infant was found to have a non-familial 47,XY chromosome complement with an extra medium-sized metacentric chromosome slightly larger than a number 16. By Giemsa-trypsin (G-banding) this extra chromosome was determined to be a number 9 with deletion of approximately half of the long arm at region q 22. Chromosome studies on the clinically normal 38-year-old mother showed a balanced translocation with the deleted portion attached onto the distal end of a number 8 short arm, i.e. 46,XX,t(8;9)(p23;q22). Nondisjunction during meiosis of this woman's normal and deleted number 9 chromosomes is the basis of the child's abnormalities. One half-sibling of the child has a balanced translocation similar to that in the mother. Chromosome analyses on 4 others of the child's maternal half-siblings and on the maternal grandmother all showed normal patterns.