HIV(+) progressive multifocal leukoencephalopathy (PML) patients had a significantly lower expression of CD62L on CD4(+) T cells (P < 0.001) when compared with HIV(+) patients who did not develop PML. CD62L expression on CD4(+) T cells did not correlate with parameters such as CDC stage, CD4(+) cell percentage (of total CD3(+) T cells), CD4(+) cell counts, virus count, or clinical parameters. Measurement of CD62L might provide a biomarker for PML risk and could prompt a treatment change and/or close monitoring.