Uvulopalatopharyngoplasty (UPPP) is the most common surgical procedure used in the treatment of snoring and obstructive sleep apnea syndrome (OSAS). However, the success of this treatment is unpredictable and much variable results were obtained by many authors. Many tests with prognostic implication for UPPP were suggested such as cephalometry, polysomnography, multiple sleep latency test, CT and so on. Many factors were involved in the pathophysiologic mechanisms of snoring and OSAS and there were much difficulties in establishing selection of patients for UPPP. There was need for simple, effecitve and economic method to select candidates for UPPP. The authors would hypothesize that possible cause for unpredictable success rate of UPPP is the variability in the site of obstruction in patients with snoring and OSAS. Under this hypothesis, we performed pharyngoscopy during Mueller maneuver and evaluated its predictive value in prognosis in 39 consecutive snoring and OSAS patients who underwent UPPP. The results were as follows : 1) The most common level of obstruction was retropalatal area (54%). 2) There was a significant correleation between AI(apnea index) and severity of obstruction during Mueller maneuver (Spearman correlation coefficients=0.58, p<0.05). 3) Postoperative improvemment rates of symptoms were 95% in retropalatal group, 80% in retrolingual group, 0% in prelingual group. There was a significant difference in improvement rates with respect to level of obstruction (p<0.05). Flexible pharyngoscopy combined with Mueller maneuver would be beneficial in selecting the candidates for UPPP by its high predictive efficacy.