Hypertension is difficult to treat in patients with type 2 diabetes mellitus (T2DM) or obesity. Combination therapies are often required to effectively lower blood pressure (BP) and attain BP goals. In this post-hoc analysis of 2 prospective, randomized, controlled studies in patients with uncontrolled or untreated moderate or severe hypertension, the efficacy and safety of treatment with irbesartan/hydrochlorothiazide (HCTZ) and irbesartan was assessed in 2 separate analyses: patients with diabetes (n=143) and by obesity status (n=1125). Patients received irbesartan/HCTZ (150 mg/12.5 mg titrated to 300 mg/25 mg) or irbesartan (150 mg titrated to 300 mg) for 7 (severe hypertension study) or 12 (moderate hypertension study) weeks. Efficacy comparisons between treatment groups were performed using Fisher's exact tests. After 7 to 8 weeks of treatment, systolic BP (SBP)/diastolic BP (DBP) decreased in patients with diabetes by 26.9/17.8 mm Hg and 21.8/15.8 mm Hg after irbesartan/HCTZ and irbesartan treatment, respectively (P [SBP]=0.09, P [DBP]=0.27). In obese patients (n=544), SBP/DBP decreased by 29.4/20.2 mm Hg and 20.1/15.9 mm Hg after irbesartan/HCTZ and irbesartan treatment, respectively (P<0.0001). More patients with T2DM reached the BP goal of <130/80 mm Hg at week 7 to 8 in the irbesartan/HCTZ group than in the irbesartan group (12% vs 5%), although not statistically significant (P=0.22). Significantly more obese patients reached their respective BP goals in the irbesartan/HCTZ group than in the irbesartan group (48% vs 23%; P<0.0001). Treatment-emergent adverse event rates were similar between treatment groups regardless of the presence of diabetes or body mass index (BMI) status. In patients with moderate or severe hypertension and with a BMI ≥ 30 kg/m(2), initial treatment with irbesartan/HCTZ combination therapy was more effective than irbesartan monotherapy.