QT interval has been determined in 103 monozygotic (MZ) and 198 dizygotic (DZ) female twins. Mean values of corrected QT (QTc) were almost identical for both groups at 413 ms (MZ) and 412 ms (DZ). There was a significant age and environmental effect on QT interval. Heritability explained about 25% of the variation in QT. Further research will show whether any of the genes known to cause Long QT syndrome have any effect on QT interval at the sub-clinical level. Journal of Human Hypertension (2000) 14, 389-390