Clinical, hematological, and cytogenetic data of 32 patients with loss of part of the short arm of chromosome 9 (9p-) are reviewed. There were 20 acute lymphoblastic leukemia (ALL), seven non-Hodgkin lymphoma (NHL), three acute myeloid leukemia, one refractory anemia with excess blasts in transformation, and one chronic myeloid leukemia (CML) in blast crisis. The cytogenetic findings were heterogeneous: 13 cases of del(9)(p21), among them four as sole karyotypic change; five cases of del(9)(p12), three of them as sole karyotypic change; four patients with i(9q), three with unbalanced translocations involving 9p12; and seven with unbalanced translocations involving 9p21. In addition, 10 patients showed known specific translocations for determined subgroups of ALL, NHL, and CML. The immunological phenotypes in the 20 ALL patients were common ALL (35%), pre-B-ALL (35%), B-ALL (5%), T-ALL (15%), and null ALL (10%). Three NHL were of T cell origin and the others of B cell origin. No specific association between the karyotypic change, immunophenotype, and clinical presentation could be ascertained for patients with ALL, acute myeloid leukemia, CML in blast crisis, and B-NHL. In T-NHL, three children with deletion of 9p, T immunoblastic lymphoma originating from common thymocyte and presenting with a mediastinal mass and pleural effusion may constitute a definite subgroup with good prognosis. All other cases had a poor outcome. Previously suggested association of 9p- with T-ALL and "lymphomatous features" was not confirmed.