CD44 is a ubiquitous cell-surface glycoprotein that displays many variant isoforms (CD44v) generated by alternative splicing of exons 2v to 10v. The expression of variant isoforms is highly restricted and correlated with specific processes, such as leukocyte activation and malignant transformation. We have herein studied CD44v expression in acute myeloid leukemia (AML) and, for comparison, in normal myelopoiesis. Protein expression of total CD44 and of CD44-3v, -6v, and -9v isoforms has been measured using specific monoclonal antibodies and flow cytometry. The composition of variant exon transcripts has been analyzed by semi-quantitative reverse transcriptase-polymerase chain reaction followed by Southern hybridization with exon-specific probes. Our data show that (1) CD44-6v isoforms are expressed on 12.0% +/- 2.5% of normal CD34(+) cells; this expression is sharply upregulated through monopoiesis and, inversely, downregulated during granulopoiesis. Also, CD44-3v and CD44-9v isoforms are detected on 10% and 14% of normal monocytes, respectively. (2) Sixty-nine from a total of 95 AML patients display a variable proportion (range, 5% to 80%) of CD44-6v+ leukemic cells. (3) A shorter overall survival characterizes the group of AML patients displaying more than 20% of CD44-6v+ leukemic cells (8 months v 18 months, P < .02). These data suggest, for the first time, that the protein expression of CD44-6v containing isoforms may serve as a new prognostic factor in AML.