Obstructive sleep apnoea (OSA) plays a significant role in increasing blood pressure. Significant decreases were reported in blood pressure of hypertensive OSA patients with sleepiness who underwent continuous positive airway pressure (CPAP) treatment, but not in non-sleepy hypertensive OSA patients. More recently, however, significant decreases in blood pressure in non-sleepy hypertensive OSA patients following CPAP were shown. Effects of sleepiness on hypertension in OSA patients have been investigated, but not the effects of hypertension on sleepiness in OSA patients. We investigated the relationships between hypertension and sleepiness in patients with OSA. We analysed data on 275 middle-aged male subjects from a cross-sectional epidemiological health survey. We measured blood pressure and sleep duration objectively using an actigraph for 7 days and the respiratory disturbance index (RDI) with a type 3 portable device for 2 nights, and assessed sleepiness using the Epworth Sleepiness Scale (ESS). The RDI correlated significantly with ESS scores in the 88 hypertensive subjects (r = 0.33, P = 0.0024), but not in the 187 non-hypertensive subjects (r = −0.01, P = 0.91). Short sleep duration correlated significantly with ESS scores in both groups. Both the RDI and short sleep duration were related independently to sleepiness in only hypertensive subjects. Furthermore, the RDI was related negatively significantly to sleep duration in hypertensive subjects. Although short sleep duration was related significantly to sleepiness in both groups, hypertension may be important for the sleepiness in OSA patients. Detailed mechanisms of the difference in the relationship between sleepiness and the severity of OSA with or without hypertension should be studied further.