BCL-1 rearrangement (BCL-1/IgH gene rearrangement) in acute lymphocytic leukemia and its clinical significance was investigated. In 38 patients with acute lymphocytic leukemia (ALL), the genomic DNA of mononuclear cells isolated from peripheral blood and bone marrow was amplified by using hemi-nested polymerase chain reaction (PCR) technique and the expression of cyclin D1 protein of mononuclear cells was detected by using immunohistochemical method. Ten patients with acute granulocytic leukemia, 2 with chronic granulocytic leukemia and 10 with normal bone marrow served as control group. The results showed that BCL-1 rearrangement was detectable in 3 of 38 ALL patients (7.9%) and cyclin D1 protein positive expression was detected in 4 ALL patients (10.5%). Three ALL patients with BCL-1 rearrangement were all B-cell leukemia (B-ALL) and accompanied by cyclin D1 protein expression. No BCL-1/IgH rearrangement or cyclin D1 protein expression was detected in 12 patients with granulocytic leukemia and 10 cases of normal bone marrow. Leukocyte counts in peripheral blood of B-ALL patients with BCL-1 rearrangement and (or) cyclin D1 protein expression were significantly increased and the patients had bad reaction to chemotherapy. It was concluded that: 1) BCL-1/IgH gene rearrangement were detected in acute B lymphocytic leukemia; 2) B-ALL patients with BCL-1 rearrangement and (or) cyclin D1 protein expression had poor prognosis.