We examined the rates of sexually transmitted diseases (STDs) in women with borderline personality disorder (BPD) (using PDE and SCID-II) and substance abuse or dependence (SCID-I) (BPD-SUD) (N=82) compared with those with BPD-only (N=102), exploring mediators of this relationship. Participants were interviewed about STD history (gonorrhea, genital herpes, syphilis, trichomonas, human papillomavirus, and HIV), condom use, number of sexual partners, poverty, and prostitution. BPD-SUD appeared to be particularly at high risk for STDs, reporting significantly more STDs than BPD (F[1,172]=11.74, p=0.001, d=.27), particularly for gonorrhea, trichomonas, and human papillomavirus. The relationship between BPD-SUD and STDs is mediated by poverty, prostitution in the last year, recent unprotected sex with two or more partners, and >20 lifetime partners (z=-2.16 which is p=0.03), with prostitution alone making a significant contribution to this relationship (z=-2.49, p=0.01).