CGL is a highly specific disease that is defined by strict hematologic parameters that include a pathognomonic differential leukocyte count. Usually CGL is accompanied by the presence, in bone marrow cells, of the Ph chromosome, the first chromosomal anomaly to be regularly associated with a human neoplastic disease. CGL is predominantly a disease of the productive middle years of life, which maximizes its adverse impact on family life and family economics. The disease is of worldwide distribution and there is a slight male preponderance. The disease is characterized by an initial chronic phase when it behaves as a differentiated neoplasm and responds very well to simple, nonintensive therapy. After a variable interval, CGL undergoes metamorphosis to a refractory phase that responds poorly or sometimes not at all to therapy, even when this is intensive. At the stage of metamorphosis a great variety of clinical and hematologic pictures occur, and CGL may mimic a myeloproliferative disease, a myelodysplasia, a subacute leukemia, AML, or ALL. The old concept of an abrupt, explosive transition from the chronic phase to a so-called blastic crisis is incorrect: this rarely occurs and in most patients who are carefully followed, CGL is observed to undergo two or more stepwise evolutions, eg, from chronic phase to an accelerated myeloproliferative phase to a phase that resembles AML. Many patients with CGL conform to an established pattern of clinical features. There is a history of insidious symptoms of anemia and of splenomegaly. The physical signs are those of pallor and marked splenomegaly, while the hematologic findings are of moderate anemia, moderate thrombocytosis, and a marked granulocytic leukocytosis with a specific differential count. The radiologic findings are typically normal. Diagnostic difficulty seldom arises with this classic presentation. The patient who is detected at an early stage of CGL may lack the history, physical signs, and fully developed hematologic picture of CGL. Before the availability of cytogenetic studies, the diagnosis could only be established with confidence by observing the patient until the typical features of the disease emerged. Also considered are the less frequent but important atypical presentations of CGL. The symptoms and complaints, findings on examination, complications and hematologic findings may depart from the typical case in a bewildering variety of ways, so that the diagnosis may be difficult, indeed, CGL is generally not the initial diagnosis that is made. When the patient with CGL has received treatment, it is usual for he or she to become asymptomatic, with no abnormal physical signs.(ABSTRACT TRUNCATED AT 400 WORDS)