The effect of infection with some viruses on the resistance to potato leafroll virus (PLRV) was studied in clones with different levels of resistance. Clones known to have high levels of resistance to PLRV infection showed a reduction of their resistance in plants previously infected with potato viruses X and Y (PVX and PVY). However, pre-infection with potato virus S did not alter their level of resistance to PLRV infection. Conversely, in clones hypersensitive to PVX and PVY, prior infection with PLRV abolished the typical necrotic reaction and the plants were not infected with either PVX or PVY. Increase of susceptibility to PLRV infection caused by pre-infection with PVX and PVY seems to be a general phenomenon among potato cultivars that becomes more evident in resistant genotypes. The practical importance of PVX and PVY infections during selection of PLRV resistant genotypes is discussed. The stability of PLRV resistance under field conditions seems dependent upon the cultivar and type of resistance to PVX and PVY. © 1989 Springer.