Low language ability and response bias are frequently cited as impediments to valid responding to items on interview schedules. Structured interviews with a random sample of 154 adults with mental retardation showed that around two thirds of respondents were either unable to respond or exhibited response bias. There was a significant difference in scores on the Adaptive Behavior Scale between those who exhibited response bias and those who did not. In cases of nonresponse or response bias, the substitution of the respondent with a proxy respondent has been proposed as an alternative method of gathering subjective data. In this study concordance between individuals' responses and those of proxies was low on a subjective measure and high on an objective scale.