OBJECTIVE: Gamma-aminobutyric acid (GABA) exerts a prominent effect on central adrenergic stress responses in times of high stress and has been associated with acute posttraumatic stress disorder (PTSD). The authors examined the association between low posttrauma plasma GABA levels and long-term PTSD. METHOD: Plasma GABA levels were measured in 78 victims of road traffic accidents who met criteria for trauma exposure on arrival at a trauma department and were admitted for at least 3 days. Patients were assessed for PTSD and major depressive disorder at 6-week and 1-year follow-ups. RESULTS: At 6 weeks and at 1 year, mean posttrauma GABA levels were significantly lower among subjects who met all or nearly all criteria for PTSD than among those who did not. Among patients who met all or nearly all criteria for PTSD at 6 weeks, 75% of those with posttrauma GABA levels above 0.20 mmol/ml no longer met criteria at 1 year. By contrast, among patients whose GABA levels were below 0.20 mmol/ml, 80% met all or nearly all criteria for PTSD at 1 year. Two-thirds of patients who met all or nearly all criteria for PTSD at 1 year also met criteria for major depressive disorder. CONCLUSIONS: A plasma GABA level above 0.20 mmol/ml may protect against chronic PTSD and may represent a marker of recovery from trauma.