The purpose of this clinical research is to learn if severe hypertension can be better controlled by initially treating with a combination of drugs (Irbesartan/HCTZ), in patients who are unlikely to achieve blood pressure (B/P) control with only one drug. In addition, the study will also evaluate the safety and tolerability of the drugs.

Inclusion Criteria:

• Men and women ages 18 and older;
• Willing to provide written informed consent;
• Must have uncontrolled hypertension defined as: currently untreated with a diastolic blood pressure greater than 110 mmHg OR currently receiving antihypertensive monotherapy with a diastolic blood pressure greater than 100 mmHg. Monotherapy is defined as treatment with one antihypertensive medication for at least four weeks; fixed combination therapy does not represent monotherapy;
• Must be willing to discontinue antihypertensive medication, if applicable;
• Women of childbearing potential (WOCBP) must be using an adequate method of contraception to avoid pregnancy throughout the study and for up to one week after the study in such a manner that the risk of pregnancy is minimized.
• WOCBP must have a negative serum or urine pregnancy test within 72 hours prior to the start of study medication.

Exclusion Criteria:

• Women who are pregnant or breastfeeding;
• Known or suspected secondary hypertension;
• Hypertension emergencies or stroke within the past 12 months;
• Heart attack, angina or bypass surgery within the past 6 months;
• Significant kidney disease;
• Significant liver disease;
• Systemic lupus erythematosus;
• Gastrointestinal disease or surgery that may interfere with drug absorption;
• Cancer during the past five years excluding localized squamous cell or basal cell carcinoma of the skin;
• Currently pregnant or lactating;
• Mental condition (psychiatric or organic cerebral disease) rendering the subject unable to understand the nature, scope, and possible consequences of the study;
• Drug or alcohol abuse within the last five years;
• Known allergy to irbesartan or diuretics.