Background: Previous studies have yielded disparate results regarding the effect of obstructive sleep apnoea (OSA) syndrome on left ventricular (LV) function. Objectives: In order to clarify this, we performed a prospective study investigating OSA patients with no history of systemic hypertension, coronary artery disease, myocardial, pericardial or valvular problems, asthma or chronic obstructive pulmonary disease before and after treatment with nasal continuous positive airway pressure (nCPAP). Methods: Fifteen patients (3 women, 12 men) with an apnoea/hypopnoea index >15 (mean ± SD = 52 ± 21) were studied with complete polysomnography, ambulatory blood pressure monitoring, M-mode two-dimensional echocardiography and pulsed Doppler echocardiography in two phases, i.e. before and after 12–14 weeks of nCPAP therapy. We measured systolic and diastolic blood pressure (BP) separately in the daytime and night-time, isovolumic relaxation time (IVRT), the ratio of peak early filling velocity (E) to peak late velocity (A) diastolic transmitral flow (E/A), posterior wall thickness (PWT) and septal thickness (IVST). The shortening fraction (SF) was also calculated. Eleven overweight non-apnoeic normal subjects matched for age were used as the control group. Results: Our results showed that the patient group exhibited, before treatment, LV diastolic, but not systolic, dysfunction compared with the normal group (IVRT = 94.3 ± 11.6 ms, p < 0.05; E/A = 0.94 ± 0.26, p < 0.02; SF = 39.9 ± 4.1%, not significant (NS); IVST = 9.9 ± 1.2 mm, NS; PWT = 8.3 ± 1.2 mm, NS). Moreover, the patient group developed diastolic hypertension both in the daytime and night-time (BP/diastolic/daytime = 93.3 ± 9.2 mm Hg, BP/diastolic/night-time = 90.3 ± 10.7 mm Hg). After 12–14 weeks of nCPAP treatment (no change in body mass index), significant improvement in LV diastolic function and a drop in blood pressure were noticed (IVRT = 85.6 ± 8.8 ms, p < 0.05; E/A = 1.07 ± 0.3, p < 0.05; BP/diastolic/daytime = 86.3 ± 5.5 mm Hg, p < 0.02; BP/diastolic/night-time = 83.9 ± 8.6 mm Hg, p < 0.05) in our patient group. Conclusions: We conclude that repetitive apnoeas/hypopnoeas are very important factors in the development of both LV diastolic dysfunction and diastolic systemic hypertension in patients with OSA syndrome. Treatment with nCPAP leads to significant improvement in both ventricular function and systemic hypertension.