Objective - To determine incidence of the lle118Lys endothelin receptor B (EDNRB) mutation responsible for overo lethal white syndrome (OLWS) and its association with specific types of white patterning. Animals - 945 horses of white-patterned bloodlines and 55 solid-colored horses of other breeds. Procedure - Horses were genotyped by use of allelespecific polymerase chain reaction to determine incidence of the lle118lys EDNRB mutation. Results - Genotypes detected were homozygous lle118, homozygous Lys118, and heterozygous. All foals with OLWS were homozygous for the lle118Lys EDNRB mutation, and adults that were homozygous were not found. White patterning was strongly associated with EDNRB genotype. Color patterns with highest incidence (> 94%) of heterozygotes were frame overo, highly white calico overo, and frame blend overo. White-patterned bloodlines with lowest incidence of heterozygotes (< 21%) were tobiano, sabino, minimally white calico overo, splashed white overo, nonframe blend overo, and breeding-stock solid. The mutation was not detected in solid-colored horses from breeds without white patterning. Conclusions and Clinical Relevance - In homozygotes, the lle118Lys EDNRB mutation causes OLWS. In heterozygotes, the mutation is usually responsible for a frame overo phenotype. The frame pattern can be combined with other white patterns, making accurate estimation of EDNRB genotype by visual inspection difficult. Wide range of incidence of heterozygotes in various subtypes of white-patterned horses indicates different genetic control of these color patterns. Determination of EDNRB genotype by use of a DNA-based test is the only way to determine with certainty whether white-patterned horses can produce a foal affected with OLWS.