The study involved 92 participants who had participated both in sex work and problematic drug use. They were singled out from other sex workers because of their drug use, and because the study was interested only in those sex workers vulnerable to being trapped in mutually reinforcing aspects of sex work and problematic drug use. Data were collected in interviews conduced between April 2001 and May 2002. Quantitative data were obtained on drug use, sex work, use of services, and offending. Three-fourths of the participants had used drugs before age 18, and half had sold sex before age 18. Half had received some type of social services; half had been homeless; 21 percent had run away or left home before age 16; and 80 percent had been convicted of acquisitive offenses. There were strong links between being trapped in mutually reinforcing sex work and drug use and convictions (81 percent), as well as between being involved in outdoor/drift sex work and being trapped. After adjustment for the other vulnerabilities in the logistic regression analysis, only outdoor/drift sex work remained significant for being trapped in mutually reinforcing sex work and problematic drug use. Since the sample overrepresents the extent of problematic drug use among indoor sex workers, the study underestimates differences in trapping potential among sex markets. The authors conclude that outdoor/drift sex markets may reinforce vulnerability, sex work, and problematic drug use. Interventions are recommended to isolate factors involved in sex and drug markets, so as to reduce the sex industry's potential for exploitation and abuse by drug marketers. 3 tables and 52 references