Sixteen patients, 15 adults and one child, with APL have been studied cytogenetically; 14 of these had an abnormal karyotype (87%). Eleven of these consistently showed a t(15;17)(q26;q22) structural anomaly, one patient showed a 47,+8 karyotype, one a rearrangement of chromosomes No. 15 and No. 17, apparently different from that in the other patients, and one a No. 17 deletion without a demonstrable translocation. as an additional chromosome change trisomy No. 8 was found in 5 cases and monosomy No. 7 in two. The t)15;17)(q26;q22) structural anomaly is highly characteristic of APL, is found in APL of children and adults, but it is apparently not associated with a clinically different form of APL.