We performed cytogenetic studies on 58 patients with hairy cell leukemia (HCL) from 1975 to 1981. Analysable metaphase cells stained with Q-banding were obtained in 77 samples from 44 patients. Cells with abnormal chromosomes were found in both unstimulated and stimulated cultures of bone marrow and peripheral blood. Patients were classified in 6 groups. Group I, 2 patients with a clonal chromosome abnormality; group 11, I3 patients with nonclonal structural abnormalities; group 111, 5 patients with nonclonal numerical abnormalities; group IV. 19 patients with only a normal karyotype; group V, 15 patients with no or with fewer than 5 normal metaphase cells; group VI, 4 patients with questionable abnormal chromosomes. Common abnormalities were deletion of the long arm of No.
6 or + 3 each in 3 patients, and + Y , + 12 or + 18 in 2 patients. Actuarial survival for each group was calculated from diagnosis and also from chromosome examination. The two patients with a clonal chromosome abnormality died within one year. Eight of 13 patients with nonclonal structural abnormalities died within 5 years after diagnosis, while none of 5 patients with nonclonal numerical abnormalities and 2 of 19 patients with normal chromosomes died within 5 years. The difference in the 5-year actuarial survival between patients with nonclonal abnormalities (groups I1 and 111) and those with a normal karyotype was significant (p <0.05). The difference was more marked between patients with nonclonal structural abnormalities and those with a normal karyotype (p <O.O I). Patients with nonclonal numerical abnormalities had a longer survival than those patients with nonclonal structural abnormalities (p < 0.05). Thus, structural chromosome abnormalities in HCL may be a poor prognostic sign even when they are not clonal.