While most forms of hypertension have no known underlying cause (and are thus known as "essential hypertension" or "primary hypertension"), in about 10% of the cases, there is a known cause, and thus the hypertension is secondary hypertension (or, less commonly, inessential hypertension).
Kidney disease / renal artery stenosis: the normal physiological response to low blood pressure in the renal arteries is to increase cardiac output (CO) to maintain the pressure needed for glomerular filtration. Here, however, increased CO cannot solve the structural problems causing renal artery hypotension, with the result that CO remains chronically elevated.
Cancers: tumours in the kidney can operate in the same way as kidney disease. More commonly, however, tumors cause inessential hypertension by ectopic secretion of hormones involved in normal physiological control of blood pressure.
Drugs: In particular, alcohol, nasal decongestants with adrenergic effects, NSAIDs, MAOIs, adrenoceptor stimulants, and combined methods of hormonal contraception (those containing ethinyl-estradiol) can cause hypertension while in use.
ABPM (ambulatory blood pressure monitoring) (in the routine evaluation and management of children with hypertension) 
Children with secondary hypertension were found to have significantly higher diastolic BP loads during both the daytime and nocturnal portions of the ABPM studies and also a significantly greater nocturnal systolic BP load, compared with children with primary hypertension. 
Poor response to therapy (resistant hypertension)
Worsening of control in previously stable hypertensive patient
Stage 3 hypertension (systolic blood pressure > 180 mm Hg or diastolic blood pressure >110 mm Hg)
Onset of hypertension in persons younger than age 20 or older than age 50
Significant hypertensive target organ damage
Lack of family history of hypertension
Findings on history, physical examination, or laboratory testing that suggest a secondary cause 
The sixth report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC-VI) defines four goals for the evaluation of the patient with elevated blood pressure:
detection and confirmation of hypertension;
detection of target organ disease (e.g., renal damage, congestive heart failure);
identification of other risk factors for cardiovascular disorders (e.g., diabetes mellitus, hyperlipidemia);
detection of secondary causes of hypertension.
^ Secondary Hypertension. Hypertension Etiology & Classification - Secondary Hypertension. Armenian Medical Network (2006). Retrieved on 2007-12-02.
^ "The sixth report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure, 1997.". Joint National Committee (Arch Intern Med 1997;157:2413-46).
^ ab Differentiation Between Primary and Secondary Hypertension in Children Using Ambulatory Blood Pressure Monitoring. American Academy of Pediatrics (2002). Retrieved on 2007-12-02.
^ Hypertension diagnosis and treatment. Health care guideline (2002). Retrieved on 2007-12-02.