Studies indicate that the serotonin system, particularly the serotonin transporter (5-HTT), may modulate the central effects of cocaine. We investigated whether a polymorphism in the 5′ promotor region (5-HTTLPR) of the 5-HTT gene confers susceptibility to cocaine dependence. One hundred and ninety-seven cocaine-dependent African-American subjects and 101 controls were studied. Polymerase chain reaction based genotyping of a biallelic repeat polymorphism in the 5′ promotor region yielded 2 alleles containing 484 (S) and 528 bp (L) repeats, respectively. There were no significant differences between controls of European background (n = 40) and African-American controls (n = 61) in distribution of genotypes (European: LL = 32.5%, LS = 40.0%, SS = 27.5%; African-American: LL = 27.9%, LS = 57.4%, SS = 14.7%) (χ2= 3.60, df = 2, p = 0.16) or allele frequencies (European: L = 52.5%, S = 47.5%; African-American: L = 56.6%, S = 43.4%) (χ2= 2.21, df = 1, p = 0.13). When cocaine patients were compared to an ethnically diverse control group (n = 101), frequencies of the L variant (65.0%) were significantly higher while the S variant (35.0%) was less frequent among cocaine patients compared to controls (L = 53.9%, S = 46.1%) (χ2= 6.83, df = 1, p < 0.01). Similarly, there were more cocaine patients with the LL genotype (41.1%) and less with the SS genotype (11.2%) compared to controls (LL = 29.7%, SS = 21.8%) (χ2= 7.43, df = 2, p < 0.05). However, after restricting controls to African-American individuals only (n = 61), cocaine subjects and controls did not differ significantly with respect to genotype distribution (χ2= 4.24, df = 2, p = 0.12) or allele frequencies (χ2= 2.83, df = 1, p = 0.10). In conclusion, although comparisons with a heterogeneous control group indicated a possible association between allelic variants of 5-HTTLPR and cocaine dependence among African-American cocaine subjects, this relationship was not observed when the control group was limited to African-American people only. Our findings need to be confirmed on larger samples of ethnically matched individuals.