Participants were interviewed at baseline, 6 months (49 percent), and 12 months (57 percent). At baseline, exchangers were less likely than nonexchangers to share syringes (21 percent vs. 37 percent), have multiple sharing partners (7 percent vs. 19 percent), and reuse their syringes (73 percent vs. 90 percent). These differences persisted in multivariate analysis adjusting for gender, age, race, education, and injection frequency. For the subset of participants who provided data at all three time points, rates of syringe sharing, syringe reuse, and indirect sharing decreased over time for both exchangers and nonexchangers. Nonetheless, levels of indirect sharing remained high among exchangers in this study and decreased significantly less than nonexchanger levels over time. This study found that exchangers and nonexchangers were reporting significant decreases in risk activities in San Francisco between 1993 and 1994. This was encouraging and indicated that IDUs had continued to reduce their risk of HIV infection through behavior modification. This study also found that exchangers reported consistently high levels of sharing cotton, cookers, or water. This was a troubling finding and suggested the need to address this risk behavior among San Francisco IDUs, particularly through syringe exchange programs. 1 figure, 2 tables, and 66 references.