THE DIAGNOSIS and treatment of hypertension is often accompanied by anxiety that may be manifested by recurrent episodes of acute hyperventilation, with symptoms that may interfere with the control of the disease. This study was designed to establish the prevalence and nature of anxiety-induced hyperventilation in a large group of difficult-to-control patients with hypertension. A prospective survey on the prevalence of anxiety-induced hyperventilation was performed with 300 consecutive adult patients referred to the author at a tertiary care referral clinic for difficult-to-treat primary (essential) hypertension. In those with suggestive episodic symptoms, voluntary hyperventilation was performed to validate the diagnosis. Of the 300 referred patients, 104 were suspected of having hyperventilation from preexisting episodic symptoms. Of the 104 patients, 88 reproduced their typical symptoms by the hyperventilation test. Eight did not reproduceall their usual symptoms and 8 others did not develop any symptoms during the 3-minute test. Anxiety-induced hyperventilation is common