A 10-month-old boy suspected of genetic abnormality was admitted for fever and coughing. Routine G-banding chromosome analysis of the peripheral blood lymphocytes and hereditary pattern analysis on the basis of the karyotypes and disease history revealed that the karyotype of the boy's mother was 46, XX,t(4;9)(q31;p24), and that of the boy was XY,der(9)t(4;9)(q31;p24)mat. The mother was identified as a carrier of balanced translocation of the chromosome who gave the abnormal chromosome 9 to her son, and she had only a chance of 1:18 to have a normal offspring. This case reiterates the importance of antemarital examination and prenatal diagnosis for preventing chromosomal diseases.