The assessment of pain experienced by the patient requiring mechanical ventilation can be a challenge for critical care nurses. This study was conducted to assess the reliability and validity of two pain behavior assessment tools. Patients who met the inclusion criteria were enrolled from 13 critical care units and one long-term acute care unit within eight hospitals. A total of 200 patients were assessed over an 8-week period. The findings suggest that both tools have adequate reliability and validity. Use of a pain behavior tool is one strategy to improve assessment of pain experienced by the mechanically ventilated adult patient who is unable to provide a self-report.