Among thymoma patients with indeterminate Western blots, two had reactivity to env but not gag, and three had reactivity to gag but not env. Among blood donors, five had reactivity to env but not gag, and one had reactivity to gag but not env. Two blood donors (BD5 and BD15) had reactivity to both gag and env (but this reactivity did not meet our criteria for Western blot positivity), and one donor had reactivity only to Western blot proteins other than gag and env. Here, ERVW-1 is linked to thymoma.