Harm reduction is a controversial model for treating drug users with little formal research on its operation and effectiveness. In order to advance the field, the authors conducted participatory research with 120 clients of harm reduction using nominal group technique to develop culturally relevant outcomes to measure progress. Second, the authors conducted focus group interviews with a different group of clients to help validate the outcomes. Third, the outcomes were used in an evaluation of the largest harm reduction program in New York City with a representative sample of 260 who completed a baseline, post and six follow-up assignments. The participatory research resulted in outcomes of ten life areas important to drug users. The evaluation results showed that the program participants make positive improvements across all outcomes with the most substantial progress made in how clients dealt with drug use problems. Along with their participation in the program, progress in some outcomes was also associated with clients' type of drug use (i.e., stable vs. chaotic) where more stable drug use was associated with the kinds or numbers of services received or the length of time in the program. This was attributed to the service delivery model of harm reduction in which clients are less inclined to associate their successes with a single worker in the program or a single service or intervention received but more with their association with the program as a whole. (Contains 13 references and 3 tables.) (Author/GCP)