Assessing pain in critically ill patients, particularly in nonverbal patients, is a great challenge. In this study, we validated a behavioral pain scale (BPS) in critically ill, sedated, and mechanically ventilated patients. The BPS score was the sum of 3 subscales that have a range score of 1-4: facial expression, upper limb movements, and compliance with mechanical ventilation. Two assessors observed and scored pain simultaneously with the BPS at rest and during painful procedures. The psychometric properties of the BPS that were studied were reliability, validity, and responsiveness. We achieved 360 observations in 30 patients. The BPS was internally reliable (Cronbach alpha = 0.72). The intraclass correlation coefficient to evaluate inter-rater reliability was high (0.95). Validity was demonstrated by the change in BPS scores, which were significantly higher during painful procedures, with averages of 3.9 +/- 1.1 at rest and 6.8 +/- 1.9 during procedures (P < 0.001), and by the principal components factor analysis, which revealed a large first-factor accounting for 65% of the variance in pain expression. The BPS exhibited excellent responsiveness, with an effect size ranging from 2.2 to 3.4. This study demonstrated that the BPS can be valid and reliable for measuring pain in noncommunicative intensive care unit patients.