This study investigated the extent to which bilingual counselors initiated informal discussions about topics that were unrelated to the treatment of their monolingual Spanish-speaking Hispanic clients in a National Institute on Drug Abuse Clinical Trial Network protocol examining the effectiveness of motivational enhancement therapy (MET). Session audiotapes were independently rated to assess counselor treatment fidelity and the incidence of informal discussions. Eighty-three percent of the 23 counselors participating in the trial initiated informal discussions at least once in one or more of their sessions. Counselors delivering MET in the trial initiated informal discussion significantly less often than the counselors delivering standard treatment. Counselors delivering standard treatment were likely to talk informally the most when they were ethnically non-Latin. In addition, informal discussion was found to have significant inverse correlations with client motivation to reduce substance use and client retention in treatment. These results suggest that informal discussion may have adverse consequences on Hispanic clients' motivation for change and substance abuse treatment outcomes and that maintaining a more formal relationship in early treatment sessions may work best with Hispanic clients. Careful counselor training and supervision in MET may suppress the tendency of counselors to talk informally in sessions.