This study uses resource dependence theory to examine how the concentration of client referrals into outpatient substance abuse treatment may affect treatment comprehensiveness. Data were from the 1995, 1999/2000, and 2005 waves of a national longitudinal survey. Results from generalized estimating equation models (sample sizes from 1,350 to 1,375) indicate that more concentrated referral sources were negatively associated with three of the four indicators of treatment comprehensiveness: the percentages of clients receiving routine medical care, mental health care, and financial counseling. Substance abuse treatment programs may be focusing their treatment practices to meet the demands of key referral sources. Given the importance of comprehensive treatment for substance abusing clients, however, these findings raise concerns about the potential implications of continued industry consolidation. The authors suggest strategies for organizations as well as policy makers to mitigate possible negative effects of very high reliance on one or two referral sources.