The relative influence of body mass index (BMI) and sleep apnea (SA) on blood pressure and prevalence of hypertension was investigated in 377 consecutive subjects admitted to a sleep laboratory. Among patients, 124 (33%) had SA, 153 (41%) were obese, and 93 (25%) had systemic hypertension. Both oxygen desaturation (OD) and minimal oxygen saturation (SaO2min) were related to BMI (p < 0.001) in patients without antihypertensive medication. Systolic and diastolic blood pressures were related to BMI and disease severity. Moreover, casual blood pressure, frequency of hypertension as well as OD and SaO2min were all significantly related to age. In a multivariate logistic regression analysis, age, BMI, and SA were all identified as independent predictors of hypertension. The relative risk associated with age was 4.3 (40 to 59 yr), with obesity 2.7 (> or = kg/m2) and with SA 2.1 (> 30 desaturation/6 h). The relative risk for hypertension increased with increasing age and BMI. In SA, however, the relative risk was unchanged in patients with a higher number of desaturations (OD > or = 60, 2.2). Combined obesity and SA resulted in a 3.9 times increase in hypertension prevalence. We concluded that age, SA, and overweight represent both independent and additive risk factors for development of systemic hypertension.