Abstract Background Hypertension guidelines recommend the use of 2 agents with synergistic action when >1 agent is needed to achieve blood pressure goals. Newer antihypertensive treatment combinations include fixed-dose combinations of an angiotensin receptor blocker and a calcium channel blocker. Objective The I-COMBINE study aimed to determine whether the antihypertensive efficacy of the fixed-dose combination irbesartan 150 mg/amlodipine 5 mg (I150/A5) was superior to that of amlodipine 5 mg (A5) monotherapy in lowering home systolic blood pressure (HSBP) after 5 weeks' treatment. Methods The I-COMBINE study was a 10-week, multicenter, Phase III, prospective, randomized, parallel-group, open-label with blinded–endpoint study. The main inclusion criterion was essential uncontrolled hypertension (SBP ≥145 mm Hg at office, after at least 4 weeks of A5 monotherapy administered once daily). Patients continued to receive A5 for 7 to 10 days and were randomized to either monotherapy with A5 for 5 weeks then amlodipine 10 mg (A10) for the next 5 weeks or to a fixed-dose combination therapy (I150/A5 then I150/A10). Safety profile was assessed by recording adverse events reported by patients or observed by the investigator. Results Following enrollment, 290 patients were randomized to treatment, and 287 (mean SD age, 57.3 11.2 years; 48% male) were included in the intention-to-treat analysis: 144 patients treated with I150/A5 then I150/A10, and 143 patients treated with A5 then A10. At randomization, mean HSBP was similar in both groups: 148.5 (10.3) mm Hg in the I150/A5 group and 149.2 (9.7) mm Hg in the A5 group. At week 5, the adjusted mean difference in HSBP between groups was –6.2 (1.0) mm Hg ( P < 0.001). The proportion of controlled patients (mean home blood pressure <135 and 85 mm Hg) was significantly higher in the I150/A5 group than in the A5 group ( P < 0.001). Treatment-emergent adverse events were experienced by 13.8% of I150/A5-treated patients and 11.9% of A5-treated patients during the first 5-week period, and by 15.8% of I150/A10-treated patients and 17.0% of A10-treated patients during the second 5-week period. Two serious adverse events were reported with the fixed-dose combination; both patients recovered. Conclusions Data from this adult population with essential hypertension suggest greater efficacy with the fixed-dose combination I150/A5 over A5 monotherapy in lowering SBP after 5 weeks. Both treatment regimens were well tolerated throughout the study. ClinicalTrials.gov identifier: NCT00956644.