The trial of the European Working Party on High blood pressure in the Elderly (EWPHE) revealed an overall decrease in cardiovascular mortality and morbidity in the actively treated patients. They received as first-line drugs a combination of hydrochlorothiazide and triamterene; methyldopa was added as necessary. The present post hoc analysis examined the effect of the diuretic treatment on cardiovascular events, both when given alone and in conjunction with methyldopa, by calculating the relative hazard rates (RHR) for cardiovascular mortality and morbidity. Using the Cox proportional hazard model, compared with placebo, a 34% reduction in cardiovascular mortality in the intention-to-treat analysis was demonstrated in the diuretic (hydrochlorothiazide and triamterene) group with an RHR of 0.66 and a 95% confidence interval (CI) of 0.44-0.97; the 16% decrease in the group treated with diuretics and methyldopa was not significant (RHR, 0.84; 95% CI, 0.56-1.25). The effect of treatment in the latter combined group became significant (RHR, 0.62; 95% CI, 0.40-0.95) when all cardiovascular study terminating events were considered; they were reduced by 38%. No effect of treatment on mortality from all causes was detected.