Acute lymphoblastic leukemia (ALL) of B-cell lineage may be classified using the French-American-British (FAB) classification as L1, L2, or L3 type. L1 and L2 ALLs characteristically express terminal deoxynucleotidyl transferase (TdT) and are surface immunoglobulin (sIg)-negative. In contrast, L3 ALL is typically TdT-negative and sIg-positive. However, in a few large studies of children with ALL, sIg expression has been reported in less than 2% of L1 and L2 neoplasms. In these sIg-positive cases, IgM usually has been detected, with Ig light chain in a subset of tumors. Surface Ig expression in L1 or L2 ALL in adults is extremely rare; we found only 1 case report in the English literature. We report 6 cases of L1 or L2 ALL with an unusual immunophenotype arising in adults. In each tumor, the neoplastic cells expressed monotypic sIg light chain (4 lambda, 2 kappa) and TdT. Three tumors expressed CD34. Cytogenetic studies in 4 cases at diagnosis or relapse revealed no evidence of chromosomal translocations involving the c-myc locus, such as the t(8;14), t(2;8), or t(8;22). Three patients responded completely to chemotherapy and are alive; follow-up ranges from 18 to 57 months. Three patients died at 3, 13, and 14 months after diagnosis.