The objective of this study was to identify factors associated with the blood pressure (BP) response to spironolactone--aldosterone receptor antagonist as an add-on therapy in patients with resistant hypertension (HTN). We retrospectively reviewed the data of subjects with resistant HTN who were treated with add-on spironolactone in a large HTN clinic. A paired Student's t-test was used to assess the differences between the BP values before and during spironolactone administration, and multivariate analysis was used to assess the predictors of a satisfactory BP response (a decrease in systolic BP >10%). We analyzed the data of 48 hypertensive participants. The add-on spironolactone therapy had a significant BP-lowering effect in both systolic and diastolic BP values (P < 0.01 for both). Baseline serum potassium levels of <4.5 mEq l(-1) were associated with a satisfactory BP response (P < 0.01). Furthermore, every decrement of 1 mEq l(-1) of serum potassium was independently associated with a fivefold higher rate of achieving a satisfactory BP response to spironolactone therapy (P = 0.024). Additional factors independently associated with an improved systolic BP response were old age (P = 0.033), body mass index (P = 0.033) and high baseline systolic BP (P=0.004). Our results support the use of add-on spironolactone therapy in patients with resistant HTN who are elderly and obese and have high systolic BP and serum potassium levels <4.5 mEq l(-1).