The objective of the present study was to determine whether information about a biometrically inferred single gene with large effects on erythrocyte sodium-lithium countertransport is useful in predicting the probability of having hypertension. We used multivariate logistic regression to model the relationship between the probability of having hypertension and predictor traits in a sample of 382 unrelated adult women and 347 unrelated adult men from Rochester, Minn. First, we identified a set of demographic, biochemical, and physiological predictors. Second, we analyzed whether the relationship between the probability of having hypertension and the identified predictor traits was heterogeneous between the biometrically inferred single locus genotypes with large effects on sodium-lithium countertransport level. Third, if there was no heterogeneity, we assessed whether sodium-lithium countertransport genotypes made an additional contribution to predicting the probability of having hypertension after other predictors were considered. In women, the predictors of the probability of having hypertension were age, plasma apolipoprotein CIII, body mass index, and an interaction term involving age and body mass index. The relationship between the probability of having hypertension and the identified predictors was not heterogeneous between sodium-lithium countertransport genotypes, and genotype did not contribute to the prediction of the probability of having hypertension after the identified predictors were considered. In men, predictors of the probability of having hypertension were age, plasma levels of high-density lipoprotein cholesterol, apolipoproteins AI and CII, sodium-lithium countertransport level, and sodium-lithium countertransport genotype. The relationship between the probability of having hypertension and sodium-lithium countertransport level and age were heterogeneous between biometrically inferred sodium-lithium countertransport genotypes.(ABSTRACT TRUNCATED AT 250 WORDS)