AbstractFew studies have specifically examined defective provirus in asymptomatic human T‐lymphotropic virus Type 1 (HTLV‐1) carriers and its relation to proviral DNA loads (PVLs). To assess the significance of defective provirus in asymptomatic carriers, we examined PVLs in peripheral blood mononuclear cells of 208 asymptomatic HTLV‐1 carriers. The mean PVLs determined using primers for the pol region were less than that for the pX region in these carriers. Analysis of seven carriers with high PVLs for the pX region but lower PVLs for the pol region showed that four had single nucleotide polymorphisms of proviral genomes for the pol region and three had HTLV‐1‐infected cells with defective provirus. Three carriers with defective provirus showed high PVLs at their initial screens, and PVLs increased after a 10‐ to 12‐year interval in two carriers. Southern blot assay showed clonal expansion of HTLV‐1‐infected cells, and the predominant clones changed during the observation period. These data suggest that although HTLV‐1‐infected cells with defective provirus may have a growth advantage, the predominant clones of HTLV‐1‐infected cells do not always survive for many years in asymptomatic carriers.