We investigated the association between late blight resistance and foliage maturity type in potato by means of molecular markers. Two QTLs were detected for foliage resistance against Phytophthora infestans (on chromosomes 3 and 5) and one for foliage maturity type (on chromosome 5). The QTL for resistance to late blight and the QTL for foliage maturity type on chromosome 5 appeared to be mapped on indistinguishable positions. We were interested whether this genetic linkage was due to closely linked but different genes, or due to one (or more) gene(s) with pleiotropic effects. We therefore developed an approach to detect QTLs, in which resistance to late blight was adjusted for foliage maturity type. This analysis revealed the same two QTLs for resistance against P. infestans, but the effect of the locus on chromosome 5 was reduced to only half the original effect. This is a strong indication that the two indistinguishable QTLs for foliage maturity type and for late blight resistance on chromosome 5 may actually be one gene with a pleiotropic effect on both traits. However, there was still a significant effect on resistance against P. infestans on the locus on chromosome 5 after adjusting for foliage maturity type. Therefore we cannot rule out the presence of two closely linked QTLs on chromosome 5: one with a pleiotropic effect on both late blight resistance and foliage maturity type, and another with merely an effect on resistance. In addition, the two QTLs for resistance to late blight showed an important epistatic interaction, suggesting that QTLs for resistance affect each other's expression.