In this study we investigated the influence of bystanders on self-administered interviews asking about the use of alcohol and illicit drugs. Interview participants were adolescents and young adults living in urban and suburban areas of the United States. Participants were assigned randomly to either a computerized or a paper-and-pencil self-administered interview. Results show that the impact of bystanders during the interview varies according to the identity of the bystander, age of the person interviewed, and the mode of interview. When a parent was present during the interview, survey participants were less likely to report the use of alcohol and marijuana. The influence of parents was stronger for adolescents than for young adults. The use of computer-assisted self-administered interviewing, compared to interviews with paper-and-pencil forms, reduced the effects due to the presence of parents during the interview. The presence of siblings during the interview had a small, negative effect on reports of using alcohol or illicit drugs. Among married or cohabiting respondents, the presence of the husband, wife, or live-in partner had no influence on reports of alcohol use or drug use.