To establish a quantitative assay for telomerase activity and analyze the telomerase activity in peripheral blood mononuclear cells (PBMNC) from patients with acute leukemia, a fluorescent dye, PicoGreen, was added to the products after telomere repeat amplification protocol. The samples were excited at 480 nm and the fluorescence emission intensity was measured at 520 nm using a spectrofluorometer. Telomerase activity was detected in PBMNCs from 20 cases of normal individuals and 25 patients with acute leukemia. The results showed that the fluorescence of PicoGreen binding to double-stranded DNA specifically was enhanced with increase of DNA quantities. In conclusion, the met hod is rapid, simple and quantitative, the telomerase activities of PBMNCs from acute leukemia patients are significantly higher than that of the normal controls.