Long QT syndrome (LQT) is an autosomal dominant cardiac disease characterized by ventricular arrhythmia. A first locus for LQT has been identified on chromosome 11p15.5 (LQT1), closely linked to HRAS. To refine the location of LQT1, microsatellites were genotyped in 8 French families and the following order was determined tel-HRAS-DRD4-D11S922-D11S4046- IGF2-INS-TH-D11S1318-D11S1323-D11S1338-D11S90 9-D11S1346-cen. By haplotype analysis, 12 crossing-over events were identified in affected and unaffected subjects, delineating the LQT1 candidate region to 7 cM. This new delineated localization between D11S1318 and D11S1323 is in a more centromeric region than previously thought and is 5 cM proximal to HRAS.