Aim To investigate whether increased plasma levels of serum 25-hydroxy vitamin D ［25（OH）VD］ are associated with cardiac structural and functional abnormalities in obstructive sleep apnea-hypopnea syndrome （OSAHS） patients. Methods We documented 337 consecutive patients with OSAHS. Left ventricular hypertrophy （LVH） ［left ventricular mass index （LVMI）≥125 g/m² in male,and LVMI≥120 g/m² in female］ was determined by echocardiography. The serum 25-hydroxy vitamin D concentration was measured by using enzyme-linked immunosorbent assay （ELISA） method. Results The LVMI,prevalence rate of LVH and body mass index （BMI） were higher in the subjects with severe OSAHS （apnoea-hypopnoea index≥30/h,LVH 75%,n＝226） than in those with mild to moderate OSAHS （LVH 40%,n＝111）（P<0.01）,and serum 25（OH）VD levels were lower in the severe OSAHS groups. Although the serum 25（OH）VD level showed a negative correlation with BMI,the results of binary logistic regression analysis demonstrated that the quartile value of 25（OH）VD was an independent significant variable for the identification of LVH （adjusted OR in quartile＝4.11,95%CI: 1.15-12.70,P<0.01）,even after adjusting for obesity and other risk factors. Conclusion Decreased serum levels of 25（OH）VD do seem to reflect an increased likelihood of LVH in patients with severe OSAHS.