We investigated sexually transmissable infection (STI) prevalence in 407 drug users in three drug treatment programmes in two Texan cities and associated demographic and sexual behaviours. Data analysis focused on differences between those for whom crack cocaine was the drug of preference compared with other drugs, since crack is associated with sexual arousal and a sex for drugs economy. Data indicate that having crack as a drug of preference is significantly associated with increased levels of previous STIs, previous drug treatment, African-American race, selling or buying sex for drugs or money, and increased infection markers for syphilis, chlamydia and herpes simplex-2. Crack preference was also significantly associated with lower rates of injecting drugs or sharing injection equipment and hepatitis C infection markers. Crack preference in heterosexual respondents was significantly associated with increased partner numbers in the past four weeks, more female partners for men and more vaginal sex contacts for men. Analysis of sex differences comparing those for whom crack was the preferred versus nonpreferred drug indicated that female crack users were significantly more likely to engage in oral sex. This supports previous ethnographic data suggesting that oral sex is a common mode of sex for drugs exchange in crack houses. In 7.4% of the total sample (14.4% of the crack-preferring sample), treatable STIs were detected. These data suggest that drug users generally, and crack-using populations in particular, in drug treatment programmes should be routinely screened for STIs as an integral part of drug treatment.