Serotonin or 5-hydroxytryptamine (5-HT) is an important neurotransmitter in the central nervous system. The serotonin transporter (5-HTT) is a key regulator of the level of serotonergic neurotransmission. In the present study, the contribution of 5-HTT polymorphisms to the risk of infantile spasm (IS) and the responsiveness to adrenocorticotropic hormone (ACTH) were investigated. Two functional polymorphisms, the 44-bp insertion-deletion polymorphism in the promoter region (5-HTTLPR) and the variable number tandem repeat in the second intron (5-HTTVNTR), were genotyped in a Chinese case-control study involving 112 patients with IS and 120 controls. Genotyping yielded valid data in 111 patients and 118 controls for 5-HTTLPR and 110 patients and 118 controls for 5-HTTVNTR. The polymorphisms were not found to have an allelic or genotypic association with IS. However, responsiveness to ACTH was higher in patients who were homozygous for L (91%) than in those with S/L (56%) or S/S (60%) (P=0.017). Haplotype analysis did not improve the observed association. The results suggest that the 5-HTTLPR genotype may influence the responsiveness to ACTH. This interpretation deserves further study.