Nineteen patients with mycosis fungoides/Sezary syndrome (MF/SZ), a malignancy of the mature helper T-cell phenotype (CD4+TCR alpha beta+), were screened for clonotypic V beta expansions in peripheral blood with a multiprobe RNase protection assay. A different predominant V beta gene was identified in 9 of 14 patients with high peripheral blood CD4/CD8 ratios, whereas 4 of these patients showed T-cell expansions expressing V beta genes other than those included in the assay. In contrast, five patients with few, if any, malignant cells in the circulation had V beta expression levels similar to that in normal peripheral blood. A unique V-D-J sequence was found for each highly expressed V beta gene, thereby documenting monoclonality of the expanded T-cell populations. Polymerase chain reaction (PCR) primers specific for the D-J beta junction accurately identified the corresponding malignant clonotype in peripheral blood. The diverse TCR V beta gene usage found in these MF/SZ patients suggests that T-cell receptor (TCR) specificity has no bearing on this disease.