The aim of this exploratory study is to gain insight into the quality of translations of informal interpreters for Turkish immigrant patients consulting Dutch general practitioners. Several questions are addressed: what role does the interpreter take in the medical trialogue, and what is his/her ‘status’?; in what way does the interpreter stimulate or hinder communicative interaction?; and what kinds of miscommunication can be observed, and what are the underlying causes? Data consisted of 16 transcripts of video-recorded medical interviews. Stretches of discourse from eight interviews with good mutual understanding (externally assessed) between patient and doctor were compared with eight interviews with poor mutual understanding. The discourse analysis focused on (1) role-taking behaviour of interpreter, (2) miscommunication and its causes, (3) changes in the translation, (4) additional information and (5) side-talk activities. Findings show substantial differences between the two datasets. In the case of ‘poor mutual understanding’, the instances of miscommunication far exceeded those characterised as ‘good mutual understanding’. Content omissions and side-talk activities seemed to hinder good mutual understanding. This study contributes to knowledge about how informal interpreters stimulate or hinder the medical interaction process.