Little is known about the composition and effects of the interpersonal contexts in which epidemiological interviews are conducted. The Epidemiological Catchment Area (ECA) study affords an opportunity to examine the frequency and impact of third party presence. Almost half of the interviews at the Los Angeles site of the ECA study were conducted in the presence of a third party, typically the spouse or child of the respondent. Results indicate that third party presence is not related either to reports of symptoms or to performance on a test of cognitive impairment. Although findings suggest that respondents are not influenced by the presence of others, they may also reflect a variety of cultural, situational, and methodological processes.