Nonadherence to antihypertensive drugs is associated with adverse outcomes; however, mediators of this relationship are poorly understood. We examined the association between the International Classification of Diseases-Ninth Revision code for medical treatment nonadherence (V15.81) assigned before initiation of antihypertensive drug therapy and all-cause mortality in a large cohort of incident hypertensive US veterans. A propensity score-matched cohort of 18 822 patients (9411 patients with and without a V15.81 code) was generated based on variables predictive of the presence of the V15.81 code to assess its independent association with all-cause mortality during 3.8 years of follow-up. We used Cox models before and after adjustment for antihypertensive drug adherence (measured as the proportion of days covered) and for measures of blood pressure to determine whether the association of nonadherence with mortality was mediated through consequences of not following prescribed antihypertensive drugs. At baseline, the mean age of patients was 50.0 years, 91.4% were men, and 33.2% were blacks. The V15.81 code presence was associated with higher all-cause mortality (hazard ratio, 1.38, 95% confidence interval, 1.26-1.52; P<0.001). Adjustment for medication adherence, blood pressure levels, and blood pressure variability during follow-up did not alter the association between the V15.81 code and all-cause mortality (hazard ratio, 1.35; 95% confidence interval, 1.20-1.52; P<0.001). In conclusion, assignment of a V15.81 code before antihypertensive drug therapy was associated with higher all-cause mortality in incident hypertensive US veterans and can be useful to identify high-risk patients in administrative databases. This association was not mediated by worse adherence to antihypertensive drugs or differences in follow-up blood pressure.