Progenies from eight tetraploid potato crosses were assessed for the association between ineffective R-genes and lesion expansion rate(LER) in mm per day on detached leaves that had been inoculated withPhytophthora infestans. In two of the progenies, a significant association between ineffective R-genes and LER was measured, which could be an indication of residual effects. No association was found in the other progenies. These results and other corroborative evidence do not support an approach to plant breeding based on the stacking ofR-genes to accumulate residual resistance effects. Using the same detached-leaf test, differences in aggressiveness between two isolates ofP. infestans could be measured in all eight progenies. LER values were consistent with previously published ranges and were distributed continuously and unimodally in all progenies, suggesting that differences in relative aggressiveness of isolates were probably not affected byR-genes.