This article provides an in-depth overview of the relationship between primary hypertension and adult obstructive sleep apnea syndrome. The background data and research are taken from the English-language literature through 1993. Primary hypertension is a common cause of major medical illnesses, including stroke, heart disease, and renal failure, in middle-aged males. Its prevalence in the United States is around 20%, with the rate of newly diagnosed hypertensive patients being about 3% per year. Sleep apnea syndrome is common in the same population. It is estimated that up to 2% of women and 4% of men in the working population meet criteria for sleep apnea syndrome. The prevalence may be much higher in older, non-working men. Many of the factors predisposing to hypertension in middle age, such as obesity and the male sex, are also associated with sleep apnea. Recent publications describe a 30% prevalence of occult sleep apnea among middle-aged males with so called "primary hypertension." Is this association fortuitous, related to a high prevalence of both diseases in the same population, or is it caused by a factor common to both diseases, such as obesity? Should the diagnosis of apnea be actively sought with sleep studies in hypertensive populations? If a diagnosis of "asymptomatic" sleep apnea is made in a hypertensive person, should the apnea be treated? Current research data provide only partial answers to these and other questions regarding the association of apnea and hypertension. Logic dictates that clinically symptomatic patients in hypertensive clinics should receive appropriate evaluation for apnea, but broad populations of hypertensive individuals should not be referred for sleep studies.