The long-QT syndrome (LQT; Ward-Romano syndrome) is a cardiac disorder that is inherited as an autosomal dominant trait. Affected family members suffer from recurrent syncope and sudden death due to ventricular arrhythmias. Recently, we identified a DNA marker on the short arm of chromosome 11 (the Harvey ras-1 locus [H-ras-1]) that was completely linked to the LQT locus in one large family. In the study presented here, we performed linkage investigations on six new and unrelated families with LQT. The LQT locus was again completely linked to the H-ras-1 locus in all families examined, with a combined lod score of 5.25 at a recombination fraction of 0. This work confirms our previous assignment of the LQT locus to chromosome 11p and supports the hypothesis that LQT is genetically homogeneous. As no obligate recombinants were identified in either this or our previous study, the H-ras-1 protooncogene remains a candidate for the LQT disease gene. Identification of LQT families with locus homogeneity is an important step in the development of a refined genetic map of this locus and will help determine whether the H-ras-1 marker would be of general use for presymptomatic diagnosis of this potentially fatal, but treatable, disorder.