Twenty-five patients with loss of part or all of 17p were selected from 701 patients with hematologic malignancies karyotyped either at diagnosis and/or at relapse and/or in refractory disease. Loss of 17p resulted from partial arm deletion (eight cases), unbalanced translocation (12 cases), i(17)(q10)(five cases) or monosomy 17 (four cases). In three cases, both 17ps were missing; in one case, two sublines independently acquired loss of 17p. In eight cases, loss of 17p was confirmed as a secondary change. The karyotypes generally were complex, with an average of 4.0 structurally abnormal chromosomes in the simplest lines showing 17p loss. The incidence at diagnosis was acute lymphoblastic leukemia (ALL) 2.2%, acute myeloid leukemia (AML) 2.4%, and myelodysplastic syndrome (MDS) 3.4%. The incidence in relapse and refractory disease was ALL 8.9%, AML 3.3%, and MDS 6.3%. The increased incidence of 17p loss in relapse and refractory disease was statistically significant (p < 0.05). Loss of 17p appears to be a feature of late-stage or resistant disease in hematologic malignancy.