Thorough follow-up and tracking procedures are critical when working with hard-to-reach populations such as drug users. Attrition in research can affect internal and external validity; therefore it is particularly important to use unique and creative tracking strategies in conducting longitudinal studies. Computerized followup procedures that were used to re-locate injection drug users (IDUs) and crack cocaine users after a 4-13 year period are described. Data are from a follow-up study conducted from 1999 to 2003, which re-located subjects who were initially recruited for NIDA's Cooperative Agreement, which took place from 1990 to 1995 in Denver, Colorado. Trackers used traditional approaches as well as computerized means to locate subjects despite scant locator information. Important lessons can be learned about the use of electronic means for locating hidden populations for health sciences research.