We report in this study the use of polymerase chain reaction (PCR) for the amplification of the genomic DNA, isolated from thymic tissue, using the primers flanking HTLV-I/IItax-rexgenes and the sequence method to analyze the HTLV-Ipolsequence of 27 Italian patients with myasthenia gravis. These molecular methods showed that 92.5% of patients tested positive fortaxgene and 55% forpolgenes; 55.5% samples were positive for both thetaxgene of HTLV-I/II, and thepolgene of HTLV-I. Histologic investigation of the thymus showed that 15 samples had thymic hyperplasia, 93% tested positive for thetaxgene, and 40% tested positive for both thetaxandpolgenes of HTLV-I. In contrast, 91.6% of thymoma-positive samples were positive for tax gene I/II and 75% positive for both genes,taxandpoltype I. The sequence analysis of PCR product fortaxandpolgenes confirmed that these amplified products were HTLV-I, with minimal variations. Our date suggested that either HTLV-I or part of the virus genome is involved in the etiopathogenesis of myasthenia gravis.