To investigate circadian blood pressure profiles in normotensive patients with moderate te severe obstructive sleep apnea (OSA), in relatien te their apnea indices and left ventricular parameters, we studied twenty one male patients with clinic bleed pressures of 140/90 mmHg. They were matched for age and body mass index (BMI) and all were heavy snorers. They underwent overnight polysomnography, 24 h non-invasive arterial blood pressure monitoring (ABPM) and echocardiography. Patients with a nocturnal reductien in average daytime systolic and diastolic blood pressure of' less than 10% were considered non-dippers. Nine patients were non-clippers and 12 were clippers. In the non-dipper group asleep systolic and blood pressure were significantly higher than in the dipper group, (P< 0.0001 and P< 0.01 respectively), There were at significant differences between the apne of hypopnea (AHI) and the oxygen desaturatien (ODI) indices of the dipper and the non-dipper groups. However, left ventricular parameters were significantly higher in the non-dipper than in the dipper group: left ventricular mass index (LVMI, p< 0.001, posterier wall thickness (PWT, p< 0.00 1 ), interventricular septal thickness (IVST, P < 0.00 1 ) and relative wall thickness (RWT, P< 0.01). In this preliminary study an absent or reduced blood pressure fall during sleep in normotensive patients with moderate to severe 0SA cannot be explained by the severity of the apnea alone. Left ventricular hypertrephy, though may be a sequel te the non-dipper situation may also be the cause of it.