This study estimated the number of street injecting drug user (IDU) clients of a syringe-exchange program (SEP) who were infected with human immunodeficie- ncy virus (HIV) and hepatitis C virus (HCV). A two-sample capture-recapture method was carried out. The first capture listed all interviewed IDUs outreached for syringe exchange from April 1 to May 1, 1998; the second capture involved those outreached from May 2 to June 6, 1998. Blood spots were collected for HIV and HCV serologies. Analysis used captured probability model varying with time. We interviewed 55 IDUs in the first capture and 99 in the second; 17 participated in both samples. An estimate of 317 IDUs attending the SEP was obtained (95% confidence interval [CI] 235–467). Based on the overall seroprevalence rates for HIV (47.7%) and HCV (53.1%), it was estimated that 151 IDUs were HIV infected (95% CI 112–223) and 168 (95% CI 125–248) were HCV infected. Enumeration of IDUs associated with estimates of the total number of HIV and HCV seropositives provide a powerful tool for SEPs to help monitor the number of IDUs, to plan for provisions, and to organize the new demands on existing health facilities for HIV and HCV care.