The influence of obstructive sleep apnea syndrome (OSAS) on left ventricular function remains controversial. We examined the influence of OSAS on global left ventricular function using the myocardial performance index (Tei index) and plasma brain natriuretic peptide (BNP) level and investigated the effect of nasal continuous positive airway pressure (nCPAP) on these parameters. We obtained echocardiographic indexes including the Tei index and BNP concentrations from 27 patients with OSAS whose mean apnea–hypopnea index was 42.2 ± 21.5 events/hour and who were undergoing nCPAP and from 22 control subjects. We defined global left ventricular dysfunction (GLVD) as a Tei index ≥0.50 and high BNP as ≥20 pg/ml. Compared with controls, the Tei index of patients with OSAS was significantly increased (p <0.01) and prevalence of GLVD was high (19%, p <0.05). The correlation between the Tei index and apnea–hypopnea index was significant (r = 0.447, p <0.05). Although BNP levels were higher in patients with OSAS than in controls, the difference did not reach significance. BNP level was high in 37% of patients with OSAS and in only 9% of controls (p <0.05). The Tei index of patients with OSAS was significantly decreased after 1 month and 3 months of nCPAP (p <0.01), and prevalence of GLVD significantly decreased from 19% to 4% (p <0.05). In contrast, BNP significantly decreased at 3 months after nCPAP (p <0.05). In conclusion, patients with moderate to severe OSAS frequently have impaired global left ventricular myocardial performance, which can be reversed at the early stage after starting nCPAP.