Research with the Important People instrument has shown that social support for abstinence is related to alcohol treatment outcomes, but less work has been done on the role of network support in drug treatment outcomes. A drug and alcohol version of the Important People instrument (IPDA) was developed and administered to 141 patients in residential treatment for cocaine dependence. Three components were found, all with acceptable internal consistency: (a) substance involvement of the network, (b) general/treatment support, and (c) support for abstinence. These components and three fundamental network characteristics (size of daily network, size of network, and importance of the most important people) were investigated as correlates of pretreatment and posttreatment alcohol and drug use. The general/treatment support component and network size were inversely related to pretreatment days using drugs, whereas network substance involvement positively correlated with pretreatment drinking frequency. Size of the daily network predicted less drinking, less drug use, and less problem severity during the 6 months after treatment, whereas general/treatment support and support for abstinence did not predict outcome. Network substance involvement decreased for patients who stayed abstinent but not for those who later relapsed. Results suggest that increasing the number of people the patient sees daily while replacing substance-involved with abstinent-supportive people may improve treatment outcomes. Treatment programs may use the IPDA to identify clients most likely to benefit from changes in their social networks.