The efficiency of marker-assisted selection for powdery mildew (Uncinula necator (Schw.) Burr) resistance in grapes (Vitis L. sp.) was studied using molecular markers associated with a major QTL (quantitative trait loci) for this trait. Initially, genetic maps were constructed from a segregating population of the cross `Horizon' × Illinois 547-1 (a hybrid between V. rupestris Scheele and V. cinerea Engelm.). A major QTL from Ill. 547-1, the resistant parent, explained 41% of the variation. One RAPD (randomly amplified polymorphic DNA) marker and one AFLP (amplified fragment length polymorphism) marker, obtained by bulked segregant analysis, showed the highest association with powdery mildew resistance in the mapping population. Segregation of the QTL was followed in different crosses by CAPS (cleaved amplified polymorphic sequence) markers developed from these two markers. An allele-specific amplified polymorphism that segregates as present/absent was also developed from the CS25b locus. Powdery mildew resistance was evaluated visually on a 1 to 5 scale in four different seedling populations. Two populations originated from crosses using Ill. 547-1 as the resistant parent. Two other populations were from crosses with NY88.0514.03, a resistant seedling from the original `Horizon' × Ill. 547-1 mapping population. Segregation ratio distortions were observed in some crosses. In these cases, the allele associated with the QTL for powdery mildew resistance was less frequent than the alternate allele. In all crosses, the markers were closely associated with resistance. If selection were based on markers, the percentage of susceptible individuals (classes 4 and 5) would decrease from 24% to 52% to 2% to 18%. Selection efficiency was greatest in crosses where segregation distortion was most intense.