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 indices including the Tei-index and BNP concentrations from 27 patients with OSAS whose mean apnea-hypopnea index (AHI) was 42.2 ± 21.5 events/hour and who were undergoing nCPAP, as well as from 22 control individuals. We defined global left ventricular dysfunction (GLVD) as a Tei-index of ≥ 0.50, and high BNP as ≥ 20 pg/ml. Compared with controls, the Tei-index of OSAS patients was significantly increased (P < 0.01) and the prevalence of GLVD was high (19%, P < 0.05). The correlation between the Tei-index and AHI was significant (r = 0.447, P < 0.05). Although BNP levels were higher in OSAS patients than in controls, the difference did not reach significance. The BNP level was high in 37% of OSAS patients and in only 9% of controls (P < 0.05). The Tei-index of OSAS patients was significantly decreased after 1 and 3 months of nCPAP (P < 0.01), and the 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.