BACKGROUND:The Mallampati classification (MLPT) is normally evaluated in the sitting position. However, many patients cannot be evaluated in the sitting position for medical reasons. Thus, we compared the MLPT in sitting and supine positions in predicting difficult tracheal intubation (DTI). We hypothesized that the diagnostic accuracy of the MLPT performed in sitting and supine positions would differ. METHODS:We performed a single-center prospective observational study in adult patients who received general anesthesia and orotracheal intubation for noncardiac surgery. During the preanesthesia consultation, the MLPT in the sitting position was recorded. The day of surgery, the MLPT in the supine position and the difficulty of intubation (DTI) were recorded by an independent observer. The diagnostic performance of the MLPT for the prediction of DTI was evaluated in the sitting and supine positions through the area under the receiver operating characteristic (ROC) curve. The performance of the Naguib score in predicting DTI was calculated with the MLPT in sitting and supine positions. RESULTS:Among the 3036 patients, 157 (5.1%) had DTI. The area under the ROC curve for the MLPT in supine position (0.82 0.78–0.84) was greater than that for the MLPT in the sitting position (0.70 0.66–0.75; P < .001). The relationship between the sitting and supine MLPTs was moderate (Spearman rank correlation coefficient0.50; P < .001). The area under ROC curve for predicting DTI by the Naguib score calculated with the supine MLPT (0.78 95% confidence interval, 0.74–0.82) was greater than that for the Naguib score calculated with MLPT in the sitting position (0.69 95% confidence interval, 0.63–0.74); P < .001). CONCLUSIONS:The MLPT performed in the supine position is possibly superior to that performed in the sitting position for predicting difficult intubation in adults.