Southern Leaf Blight (SLB) is an important foliar pathogen of maize in the southeastern U.S. and in some regions of Africa, India and Western Europe where environmental conditions are warm and humid. The commonly used stiff stalk line B73 is susceptible to SLB, while sister lines NC250 and NC250A, derived from crosses between temperate and tropical germplasm, show extremely high resistance. Quantitative trait loci (QTL) associated with SLB resistance have been identified on chromosomes 1, 2, 3, 5, 6, 7, 9 and 10 in a B73 x NC250A F_2:3 mapping population. Two highly SLB-resistant B73 near-isogenic sister lines, NC292 and NC330, were derived from a B73 x NC250 cross followed by several backcrosses to B73 under recurrent selection for SLB resistance. A whole genome scan was undertaken to identify NC250 introgressions in NC292 and NC330. Results confirmed that NC292 and NC330 are ³90% B73-like and identified nine regions introgressed from NC250, with some corresponding to regions where QTL were identified in the B73 x NC250A F_2:3 population. Additional backcrossing to B73 was performed to generate lines with only one or two NC250 introgressions. After detailed genotyping in the BC₁F₁ and BC₁F₂ generations, near-isogenic lines that were homozygous for single or double NC250 introgressions were selected and BC₁F₃ lines were phenotyped in a SLB field nursery. Single introgression lines were developed for detailed characterization of all nine NC250-derived introgressions identified in the genome scan. BC₁F₃ lines containing a NC250 introgression in the region encompassing SSR markers phi099 and umc1025 on chromosome 3 were often among the most SLB resistant lines. A SLB QTL was identified in this region in this and other QTL studies. NC250 introgressions within markers umc2246 and umc2363 on chromosome 2 and markers bnlg2305 and umc2307 on chromosome 5 were often associated with the more SLB resistant lines.