As health services research focuses on prevention and drug treatment p
rograms for special populations, maintaining panel samples at adequate
levels over time becomes more important. Differential panel mortality
poses a serious threat to the internal validity of experimental desig
ns and the external validity of study conclusions. Minimizing responde
nt attrition demands systematic attention to two problems: location of
subjects at each wave and sustaining respondent cooperation over the
life of the research. Major obstacles to collecting longitudinal data
on non-traditional populations include the inapplicability of most com
munity-based and governmental records, inexperience with tracking thos
e without stable addresses, and cost. This paper describes and evaluat
es methods used to track a panel of 670 homeless substance abusers ove
r 3, 6, and 12 month intervals. Results suggest that a broad brush tra
cking strategy with emphases on telephone, mail and field tracking ser
ved to minimize panel mortality and its accompanying methodological pr
oblems.