Epidemiological data on HIV seroprevalence has been essential in assessing the (future) extent of the AIDS epidemic. By coupling these data with quantifiable variables related to injection drug use (frequency of injecting, number of needle sharing partners) specific 'risk behaviors' could be determined, accounting for the rapid spread of the virus in the injecting drug user (IDU) population. Yet, such data give little information on the social mechanisms and setting generating such risk behaviors. In order to understand the transmission of HIV among the IDU population one needs to study the micro settings and social context of drug use. This paper describes and explores certain patterns of drug use, sharing, and natural support systems found amongst IDUs in two very different cities, Rotterdam (The Netherlands) and the Bronx, New York City (USA). By specifying details of the micro-settings of everyday drug use in both locales, it is possible to identify certain common elements and consequences of personal and social behavior driven by drug use per se (e.g. drug preference), and to differentiate these from behaviors and consequences determined by drug policy and the social context in which drug use actually occurs. These policies and the social context they create can in turn be shown to relate to risks for HIV transmission, e.g. the increased likelihood of sharing injection equipment. A more careful ethnographic approach, taking advantage of natural experimental opportunities, comparisons and controls, may be utilized to examine drug-related behaviors in their social context and to better assess their relevance to public health--especially to AIDS.