Posttraumatic stress disorder (PTSD), depression, anxiety, and stress are significant problems among returning veterans and are associated with reduced quality of life.A correlational design was used to examine the impact of a polymorphism (5-HTTLPR) in the serotonin transporter promoter gene on post-deployment adjustment among returning veterans.A total of 186 returning Iraq and Afghanistan veterans were genotyped for the 5-HTTLPR polymorphism. Symptoms of PTSD, depression, general stress, and anxiety were assessed along with quality of life.After controlling for combat exposure, age, sex of the participant, and race, 5-HTTLPR had a significant multivariate effect on post-deployment adjustment, such that S' carriers reported more post-deployment adjustment problems and worse quality of life than veterans homozygous for the L' allele. This effect was larger when the analyses were restricted to veterans of European ancestry.Our findings suggest that veterans who carry the S' allele of the 5-HTTLPR polymorphism may be at increased risk for adjustment problems and reduced quality of life following deployments to war zones.