We report the clinical histories and a multiparameter pathological study of the extramedullary lesions of seven patients with chronic myelogenous leukaemia in whom the initial clinical presentation of blast crisis (BC) was in an extramedullary site (lymph nodes in six, mandibular mass in one). Bone marrow BC was demonstrated simultaneously or within a few months in four patients. Three patients received chemotherapy only, four underwent bone marrow transplant. Six patients died within 1 year from diagnosis of extramedullary BC, one is alive without disease. The longest survivals (12+, 12, 11 months) were those of patients who never developed bone marrow BC and were recipients of bone marrow transplant. Studies of extramedullary disease included: histology; histochemistry for chloracetate esterase (CAE) and lysozyme; assays for TdT; electron microscopy; immunofluorescence for Fc-receptors, immunoglobulins and lymphoid and myeloid antigens by a panel of monoclonal antibodies; and cytogenetics. Three cases were classified as myeloid BC based on histochemistry and/or ultrastructure and immunology (OKM1+, MCS2+, IG10+); two as lymphoid BC (CAE-, lysozyme-, TdT+), one of them expressing a T-cell phenotype. Cytogenetic analysis of extramedullary lesions and simultaneous or subsequent bone marrows demonstrated identical karyotypes in three patients and significantly different karyotypes in one.