Obesity induces multiple physiologic changes at the respiratory and circulatory systems level. A study was developed to identify symptoms and signs able to discriminate subjects at high risk of obstructive sleep apnea (OSA) and to evaluate the presence of OSA in a population of obese patients referred to the Clinical Nutrition Service of the Luigi Sacco Hospital for weight loss therapy. Twenty-seven obese patients (14 males, 13 females) without neurologic, cardiac, and lung diseases were measured for height, weight, neck, waist, and hip circumference; a sample of venous blood was taken for hematological data; and were given a pulmonary function test, hemogasanalysis, and full-night polysomnography. Statistical analysis were performed using paired and unpaired StudentOs t test, PearsonOs chi square, and Spearmann Rank correlation; the significance level was set at p<0.05. The results showed hemotological values in the normal range and pulmonary function findings were not different from predicted, but expiratory reserve volume (ERV), as expected in obese subjects, was significantly reduced (p<0.001). Waist, hip, and neck circumference, and waist/hip ratio were 114 +/-14, 118 +/-12, 44 +/-4, and 0.96 +/-0.4 cm respectively. An apnea-hypopnea index (AHI) cutoff value of <15 was used to classify the patients as suffering from OSA: 15 patients (12 males, 3 females, age in years 55 +/-12, body mass index (BMI) kg/m(2) 37 +/-6, AHI 30 +/-12) were OSA and 12 patients were non OSA (2 males, 10 females, age in years 49 +/-20, BMI kg/m(2) 35 +/-2, AHI 3 +/-2). PaO2 and pH were lower and PaCO2 higher in OSA (p<0.05, p<0.01, p<0.05, respectively). Red blood cells (RBC), Hb, and neck circumference were increased in OSA (p<0.05). In OSA patients, S3%, S4% of total sleep time, SaO2% mean of nadir were reduced (p<0.001), and DEF increased (p<0.0001). In obese patients, AHI was correlated with neck circumference (r = 0.74, p<0.0001) and waist/hip ratio (r = 0.48. p<0.01). DEF was correlated with RBC, Hb, Htc% (r = 0.82, 0.71, 0.66, p<0.001). SaO2;% mean of nadir was significantly related to RBC, Hb, and Htc% (r = 0.44, 0.40, p<0.05, respectively). Our data showed a prevalence of OSA in 55% of the obese patients. A significant correlation exists between RBC, Hb, Htc%, with desaturation events frequency (DEF) and SaO2% of nadir indicating that transient, episodic desaturation during sleep is linked to a moderate increase of RBC and Hb found in obese patients with OSA, in contrast to obese, nonOSA patients. The most important result of the present study was the determination that classical symptoms and signs of OSA, such as male gender, neck circumference, waist/hip ratio, RBC, and Hb at the upper limit of normal, are simple inexpensive screening tools, and useful predictors of sleep-disordered breathing and discriminate the individuals with higher risk of OSA.