ABSTRACT: OBJECTIVES:Deficient plasminogen activator inhibitor-1 (PAI-1) prevented hypertension in mice. Plasma PAI-1 was associated with hypertension in cross-sectional analyses, but the prospective association of PAI-1 with incident hypertension in large epidemiological studies is scarce. METHODS:Leveraging two longitudinal cohorts of American Indians in the Strong Heart Study (SHS, N?=?1019) and the Strong Heart Family Study (SHFS, N?=?1502), we examined the prospective association of plasma PAI-1 with incident hypertension by multivariate logistic regression, adjusting for age, sex, study site, smoking, drinking, dietary sodium, obesity, lipids, fasting glucose, kidney function, inflammation, and follow-up years. Family relatedness in the SHFS was accounted for using the GLIMMIX procedure. Plasma PAI-1 level at baseline was measured by immunoassay. All participants were free of hypertension, cardiovascular diseases, and chronic kidney disease at baseline. RESULTS:A total of 305 and 258 participants, respectively, from the SHS (57?±?7 years) and the SHFS (33?±?13 years) developed incident hypertension during follow-up. In the SHS, higher level of log-transformed PAI-1 was associated with 1.35-fold increased risk of hypertension [odds ratio (OR) (95% confidence interval): 1.35 (1.06-1.72)]. Analysis using categorical PAI-1 (in tertiles) showed that participants in the highest tertile (?58?ng/ml) had 63% increased risk for hypertension [OR?=?1.63 (1.12-2.37)] compared with those in the lowest tertile (<33?ng/ml). This association was confirmed in the SHFS with similar effect sizes [OR?=?1.41 (1.11-1.81) for log-transformed PAI-1; OR?=?1.64 (1.08-2.50) for categorical PAI-1: ?58 vs. <33?ng/ml]. CONCLUSION:A higher level of plasma PAI-1 is significantly associated with hypertension in American Indians, independent of established risk factors. The potential causality warrants further investigation.