<i>Objective:</i> It is known that a 32-bp-deleted CCR5 mutant (CCR5-Δ32) plays a critical role in resistance to human immunodeficiency virus type 1 (HIV-1) infection. We became aware of a number of seronegative patients at high risk of HIV-1 infections. Since CCR-Δ32 is not found in Thais, we examined the role of stromal cell-derived factor (SDF) 1-3′A polymorphism in resistance to HIV-1 in seronegative Thai prostitutes at high risk. <i>Methods:</i> We determined CCR5-Δ32 by PCR, SDF1-3′A by RFLP, and plasma SDF-1 level by sandwich ELISA. <i>Results:</i> We did not find any CCR-Δ32 mutant in 432 subjects. In our SDF1-3′A genotyping, we found allelic frequencies of 0.406, 0.199, and 0.289 in HIV-1-seronegative prostitutes at high risk, HIV-1-seropositive patients, and normal blood donors, respectively. There was a significant difference in the allelic frequencies of SDF1-3′A between HIV-1-seronegative prostitutes at high risk and HIV-1-seropositive patients (p < 0.005). Striking differences were seen in homozygotes (SDF1-3A/3A); 20.3% in HIV-1-seronegative patients at high risk compared with 2.5% in HIV-1-seropositive patients. In addition, we showed that the plasma SDF1 level in the HIV-1-seronegative prostitutes at high risk was twice that found in normal blood donors (p = 0.005), and even higher than that of HIV-1-seropositive patients (p = 0.002). <i>Conclusions:</i> The finding is relevant as regards the fact that SDF1-3′A polymorphism induces an increase of SDF1 chemokine production, in which it competes with HIV-1 in binding to CXCR4 receptor, and in turn inhibits HIV-1 infection. The SDF1-3′A-mediated resistant mechanism in Thais differs from that of CCR5-Δ32-mediated resistance in Caucasians. This study provides the first evidence for SDF-3′A polymorphism in resistance to HIV-1 infection in Thais, and may represent the resistant mechanism in the extremely rare CCR5-Δ32 mutant of other ethnic groups such as Africans and Japanese.