Given that single blockade with angiotensin-converting enzyme inhibitors (ACEIs) or angiotensin receptor blockers (ARBs) can achieve only partial and undurable suppression of the Renin Angiotensin System (RAS), it has been hypothesized that dual blockage would be more beneficial in the management of blood pressure (BP) reduction and prevention of progressive chronic kidney disease (CKD) than either agent alone. Thus, it has been suggested that the combination of an ACEI and an ARB might provide renal benefits to hypertensive patients over and above BP reduction. However, this might also expose patients to additive or synergistic side effects. We attempted to conduct a systematic review to evaluate the benefits and harms of combination therapy in hypertensive patients with or without kidney diseases. MEDLINE and KoreaMed were searched for relevant randomized clinical trials in adult hypertensive patients with or without diabetes (restricted to 1997, limited to trials published in English). Results were summarized using the random-effects model, and between-studies heterogeneity was estimated with $I^2$. A final analysis of ten trials (23,928 patients) revealed that the combination of an ACEI and an ARB reduced blood pressure (SBP/DBP) by 3.95/2.02 mmHg (95% confidence interval [CI], -4.38 to -3.53/-2.33 to -1.71) compared with ACEI monotherapy, and 2.83/2.64 mmHg (95% CI, -3.25 to -2.41/-4.95 to -0.33) compared with ARB monotherapy. Eight trials (391 patients) demonstrated a significant reduction in 24h-proteinuria (weighted mean difference, 0.16 g/day, 95% CI, -0.26-0.05), but they did not translate into an improvement in GFR. Tests for heterogeneity showed no difference in effect among the studies. The combination therapy reduced proteinuria by 30% (95% CI, 23% to 37%) and 39% (95% CI, 31% to 48%) compared with ACEI monotherapy and ARB monotherapy, respectively. However, in patients who had proteinuria more than 0.5 g/day, the combination therapy failed to show significant reduction in urinary protein excretion. The current cumulative evidence suggests that diabetic patients with proteinuria on dual RAS blockade have an increase risk of adverse events such as hyperkalemia, hypotension, and so on, compared with ACEI or ARB alone. It is, therefore, proposed that the combination therapy should not be routinely used for the treatment of hypertension with or without compelling indications.