['To assess whether changes in cytochrome P-450 (CYP) activity of specific CYP enzymes occur in severely injured patients and to assess changes in CYP activity during recovery.', 'Prospective clinical study.', 'University-affiliated, level I trauma center and trauma critical care unit.', 'Twenty-three multiply injured patients admitted to a trauma critical care unit were compared with healthy volunteers.', 'CYP metabolizing activity was assessed using the probe drugs mephenytoin (CYP-2C19), chlorzoxazone (CYP-2E1), dapsone (multiple CYP enzymes), and flurbiprofen (CYP-2C9).', 'Mephenytoin metabolism was profoundly suppressed after injury and increased during postinjury recovery, whereas chlorzoxazone metabolism was suppressed to a lesser degree. Measures of dapsone and flurbiprofen metabolism were elevated throughout the study. Chlorzoxazone and mephenytoin metabolism correlated with the multiple organ dysfunction score and with the multiple organ failure score.'] Is cytochrome P-450 activity differentially altered in severely injured patients?