Northern corn leaf blight (NCLB), (causal agent Exserohilum turcicum), is an important disease to maize breeders in humid and temperate areas. Maize breeders have used predominately quantitatively inherited resistance to NCLB in current hybrid development programs. There is little information regarding the location and effects of quantitative trait loci (QTL) affecting host-plant response to this disease, apart from earlier studies utilizing reciprocal translocations.;In this investigation restriction fragment length polymorphisms (RFLPs) were used to obtain more precise estimates of gene location and effects for host-plant response to NCLB. One hundred and fifty unselected F[subscript]2:3 lines from a B52/Mo17 population were inoculated with spore suspensions (race 0) of the pathogen and then rated for host-plant response, focusing on average number of lesions per leaf, average percent leaf tissue diseased (severity) and average size of lesions. Number of lesions appears to be associated with QTL on chromosomes 1S, 3L, 5S. Severity was associated with analogous regions and, in addition, QTL on chromosomes 7L and 8L. Most QTL, for either of these two components, involve additive gene action or partial dominance. In contrast, lesion size was associated with QTL on chromosomes 7L and 5L; recessive gene action may be involved at 7L. Estimates of gene location concurred with earlier reciprocal translocation mapping studies on the importance of chromosomes 3, 5 and 7, despite the fact that these studies utilized diverse sources of resistance.;The location of QTL were compared with loci having known qualitative effects, namely Ht1, Ht2 and bx1. Chromosomal regions containing the Ht1 and Ht2 loci showed a small contribution in determining lesion size, even though alleles with dominant qualitative effects at these loci have never been reported in either inbred parent. Similar effects were not observed for number of lesions or disease severity. Some contribution was observed for chromosomal regions encompassing the bx1 locus in determining lesion size but not number of lesions or disease severity. Overall the contributions of the loci in the vicinity of Ht1, Ht2 and bx1 were small compared to variation attributable to loci with quantitative effects identified in this study.