The peripheral blood lymphocyte chromosomes in a case of ⁶⁰Co γ rays accident were examined at 2.5 h after exposure. The frequency of the dicentrics plus centric rings was 89% and exposure dose was estimated to be 4.78 (4.53〜4.88) Gy. The examinations of lymphocyte chromosome aberration within follow-up 12 a showed that the incidence of Dic+R reduced with lg regression (r =-0.9895). While the number of cells with stable aberration remained unchanged and showed a tendency to increase. During the period of leukemia, bone marrow cell chromosome aberrations were studied by method of G-banding. Of 13 cells observed, 4 cells belonged to normal karyotypes. Among 9 aberration cells, 12 aberrations were detected. The majority of which were classified as translocation, deletion and inversion. Numeric aberrations were -9, -12, -20, -22, -y. This case suggested that acute lymphocytic leukemia was induced after radiation accident.