Acute leukemias (ALs) with phenotypic and genotypic features of several hematopoietic lineages are difficult to classify and may represent the transformation of multipotent stem cells. We have studied immunological features of 200 cases of acute leukemia (109 acute myelogenous leukemia, AML, and 91 acute lymphoblastic leukemia, ALL, according to FAB criteria), including 17 (8.5%) classified as biphenotypic by a scoring system based on the number and specificity of unexpected lineage antigens and which gives more weight to cytoplasmic markers such as myeloperoxidase, CD3, and CD22, and less to other membrane markers. Sixty-eight AML and 42 ALL cases were also examined for rearrangements of the immunoglobulin (Ig) and T-cell receptor (TCR) beta, gamma, and delta genes, and these included 12 biphenotypic AL. The expression of myeloid antigens in ALL was seen in 25% of the cases. All B-lineage ALL had rearrangements and/or deletions of the Ig genes whereas TCR beta, gamma, and delta genes were rearranged in 21%, 52%, and 71%, respectively. TCR delta, gamma and/or beta were rearranged in T-ALL and four out of 13 cases had Ig gene rearrangement. Lymphoid-associated antigens were expressed in 40% of AML cases; those most frequent expressed were CD7 (17%), CD2 (15%), CD19 (10%), and CD10 (7.5%). Evidence of Ig and/or TCR gene rearrangements was detected in 12% of AML cases. There was no correlation between the isolated expression of terminal deoxynucleotidyl transferase (TdT), B, and T-cell antigens with Ig and TCR gene rearrangements. However, in cases of AML defined as biphenotypic because they expressed two or more lymphoid antigens there was a statistically significant correlation between gene rearrangements and lymphoid score (p < 0.001). Our findings support the concept of biphenotypic leukemia as a distinct entity in which there is frequent correspondence between phenotypic and genotypic changes.