Methods: A consecutive sample of 200 patients attending a TBI clinic who have sustained a mild-to-moderate TBI (American Congress of Rehabilitation Medicine criteria) within the last 2 months will be assessed for the presence of major depression (standard criteria, standardized interview). In phase I, blood samples from patients with mild-to-moderate TBI with depression and without depression will be checked for the presence of the 5-HTTPR genetic difference. This will allow us to study if the S allele is more likely in TBI patients with depression. In phase II, the patients with depression will be treated with the SSRI citalopram for 6 weeks. At 6 weeks, or upon discontinuation of citalopram, depression will be assessed again. This will allow us to study if depressed patients with the S allele respond more poorly to treatment. Persons assessing depression after treatment will not know the genetic makeup of each patient.