OBJECTIVE To investigate the change of fasting serum adiponectin in obstructive sleep apnea-hypopnea syndrome (OSAHS) patients. METHOD Forty males with OSAHS and forty age-matched male normal controls were included in the study. Subjects in OSAHS group were divided into two sub-groups according to body mass index (BMI): obese OSAHS group (BMI > or =25, n=26) and non-obese group OSAHS (BMI <25, n=14). All subjects underwent an overnight sleep study. The serum adiponectin levels were measured by ELISA. RESULT (1) Except for BMI,compared with control subjects, levels of fasting adiponectin level were significantly lower in OSAHS subjects (P < 0.05). (2) In obese OSAHS sub-group, serum adiponectin level was negatively correlated with BMI and AHI. However, serum adiponectin level was positively correlated with the minimum oxygen saturation. There were similar correlations between serum adiponectin level and sleep parameters in non-obese OSAHS sub-group. CONCLUSION Despite age and BMI, fasting adiponectin level was significantly lower in OSAHS patients than that in control subjects. And fasting adiponectin level was correlated with BMI, AHI and the minimum oxygen saturation. OSAHS is one of the main reasons of the decreased adiponectin.