Association between 5-HTTLPR polymorphism and development of acute and persistence of chronic posttraumatic stress disorder (PTSD) was prospectively investigated. DNA was extracted from 41 motor-vehicle accident victims evaluated for development and persistence of PTSD, 1 and 12 months posttrauma. At Time 1, a nonsignificant trend for higher acute PTSD rate in ll homozygotes (82%) was observed compared to those with ss and sl genotypes (50%). At Time 2, higher chronic PTSD rate was found in ll homozygotes (55%) compared to those with ss and sl genotypes (20%), with an odds ratio of 4.8 (95% CI = 1.09-21.22). Contrary to previous findings, these data are suggestive of a protective role for the s allele of 5-HTTLPR in chronic PTSD.