This article discusses the changes in injectingdrug use from 1998 to 2003 in Buenos Aires, Argentina.The Rapid Situation Assessment andResponse methodology was used to obtain theinformation. Quantitative and qualitative techniqueswere triangulated: 140 current IDUs and35 sex partners of injection drug users (IDUs)were surveyed; 17 in-depth interviews with thesurveyed IDUs and 2 focus groups were held, aswell as ethnographic observations. The way inwhich risk and care practices among injectingdrug users changed and the influence of theHIV/ AIDS epidemic on this process are described.In recent years, the frequency of injectionpractices and sharing of injecting equipmenthas decreased, while injecting drug use isa more hidden practice in a context of increasingimpact of the disease in the injecting druguse social networks and changes in the price andquality of drugs. Knowledge about these changeshelps build harm reduction activities orientedto IDUs in their particular social context.