The epidemiological, clinical, hereditary, biochemical, hematological, and physiological characteristics of essential hypertension (EH) and obstructive sleep apnea (OSA) are reviewed here. This extensive review shows that essential hypertension and sleep-disordered breathing--independently of whether it is OSA syndrome or upper airway resistance syndrome--share strikingly similar characteristics. The accumulated data obtained by many different researchers support the hypothesis that EH is mainly due to increased upper airway resistance during sleep. If this hypothesis is correct, treating disorders that cause increased upper airway resistance, particularly during sleep, would be an important part of the treatment of essential hypertension.