Cost-effectiveness methods were used for a rigorous evaluation of Group Guidance, a detached worker technique used in Los Angeles for coping with delinquent gangs. One hundred core members of three comparable gangs were subjects of a study designed as a natural experiment. One gang had received full Group Guidance service, another had received partial service, and the third gang had received no service.Gang members' records were examined for correctional actions and services occurring over a six-year period which was divided evenly into "before treatment" and "after treatment" phases. The total period covered, roughly, the fourteenth through the twentieth year in the gang members' lives.By use of current auditor's estimates of the costs of actions and services, a career correctional cost was determined for each boy over the six-year follow-up. The cost data disclosed a relationship between service by detached workers and differentials in mean correctional career costs. Application of Group Guidance techni ques was associated with (1) reduction in mean correctional costs from "before" to "after," and (2) reduction in mean costs from no treatment to full treatment. For the fully treated gang, the reduction in correctional costs in one year was several times the cost of the treatment. For the untreated gang, correctional costs rose from before to after.