Background and Objectives: This study is designed to understand the pathophysiology of one of the most serious health problems, chronic tension-type headache (CTTH). Two polymorphic sites in serotonin transporter protein gene attracted much interest. These are: the variable number of tandem repeats (VNTR) and 5'-flanking promoter region (5-HTTLPR). Materials and Methods: VNTR and 5-HTTLPR polymorphisms were investigated in 126 CTTH patients and 138 healthy control subjects. The patients were being treated with amitripytyline or citalopram or sertraline (SSRI). The polymerase chain reaction (PCR) method was used to investigate the polymorphisms in the serotonin transporter protein gene. Results: There were no statistically significant results based on the 5-HTTLPR gene alleles, however, STi...