Left ventricular hypertrophy remains a significant clinical problem and a predictor of fatal outcome in hypertension. Blood pressure per se and environmental modifiers including stress affect cardiac mass. Heat shock proteins are involved in the stress response as well as in the regulation of cardiac growth and cytoprotection. The present study evaluates heat shock protein 27 as a locus marker or candidate gene of cardiac hypertrophy in hypertension. The spontaneously hypertensive rat allele of heat shock protein 27 was associated with about a 6% increase in relative left ventricular weight (P = .0112) in 30 recombinant inbred strains from crosses of Brown Norway and spontaneously hypertensive rats. In 336 F2 crosses of spontaneously hypertensive and Wistar-Kyoto rats, the hypertensive allele was dominant and cosegregated with a similar 6% increase in the ratio of left ventricular weight to body weight (P = .0058) in rats fed a normal salt diet, but its contribution to left ventricular weight decreased in rats kept on a high salt diet. The contribution of the heat shock protein 27 allele was independent of blood pressure. We suggest that heat shock protein 27 represents a candidate gene/locus marker of cardiac hypertrophy in hypertension.