In this report, we quantified HTLV-I provirus load using the AmpliSensor system, which utilizes fluorescence to measure PCR products. With this method, provirus loads could be measured within 6 hr, and the results obtained correlated well with those obtained by other methods. Samples from 256 blood donors, who were positive for antibodies against HTLV-I, were analyzed, showing that provirus load ranged from less than 0.1% to 56% among carriers. We analyzed the association between provirus load and the biomarkers age and sex and found that it was not influenced by either. Provirus load was better correlated with soluble interleukin-2 receptor (sIL-2R) levels than with antibody titer against the virus. Among 18 blood donors with high provirus load (more than 10%), Southern blotting detected monoclonal integration of HTLV-I in infected cells in 2 cases, both of them showing high sIL-2R levels (more than 900 U/ml). Sequential analyses of provirus load showed stable levels of provirus in the same carriers, suggesting that some factors other than age or sex determined provirus load in infected individuals. Thus, this rapid method is a useful tool for the early detection of adult T-cell leukemia and other HTLV-I-associated diseases.