The researchers used calendar year 2000 data from interviews with 1,440 respondents involved in the Arrestee Drug Abuse Monitoring (ADAM) program in Manhattan, Chicago, Portland (OR), and San Diego. In all cases, research teams interviewed and urine-tested a sample of arrestees who had been brought to a local lockup or booking center and booked. Interviews were conducted within 48 hours of arrest of individual respondents. Respondents were asked a number of questions about their drug market participation, including whether or not they had obtained drugs in the past month; whether or not cash or something else was used to make the purchase; how the purchase was arranged; the number of dealers the respondent used to purchase a particular drug; whether or not the respondent bought drugs outside his/her own neighborhood, indoors or outdoors, or from one regular dealer as a main source; and whether or not the respondent had experienced a failed drug transaction, and why. Responses to some of these questions can be interpreted as indicative of the extent to which the illicit drug market in which the respondent participated in the past month consisted of patterns of operation and relationships that were changing in an orderly and predictable way; for example, a structurally more stable market would have "businesslike operations with manufacturing, distribution, and trade being organized around socially controlled relationships between producers and distributors, employers and employees, and merchants and consumers" (Brownstein et al., 2000). ADAM respondents generally reported that they rarely if ever had trouble getting drugs when they wanted them. This suggests that the dealers are where the buyers expect to find them, and they have what the buyers desire. This in turn suggests that the suppliers and distributors are able to supply the dealers with the product that the buyers want and that the dealers are able to operate from their standard selling location; this would indicate a more structurally stable market. This paper thus demonstrates how it is possible to operationally define the relative stability of illicit drug markets, to construct variables that measure both structural and interactional stability, and to illustrate how such measures of stability so defined can be used to compare particular drug markets in different urban communities or over time in the same urban community. 3 tables, 7 notes, and 48 references