This article discusses the changes in injecting drug use from 1998 to 2003 in Buenos Aires, Argentina. The Rapid Situation Assessment and Response methodology was used to obtain the information. Quantitative and qualitative techniques were triangulated: 140 current IDUs and 35 sex partners of injection drug users (IDUs) were surveyed; 17 in-depth interviews with the surveyed IDUs and 2 focus groups were held, as well as ethnographic observations. The way in which risk and care practices among injecting drug users changed and the influence of the HIV/ AIDS epidemic on this process are described. In recent years, the frequency of injection practices and sharing of injecting equipment has decreased, while injecting drug use is a more hidden practice in a context of increasing impact of the disease in the injecting drug use social networks and changes in the price and quality of drugs. Knowledge about these changes helps build harm reduction activities oriented to IDUs in their particular social context.