Against the background of current hospital interpreting practices in Vienna, the authors present a case study of an authentic therapeutic interaction in which a Serbian-speaking hospital cleaner serves as interpreter in a 47-minute voice therapy and briefing session. Communication between the two speech therapists and the ten-year-old voice patient and his parents from former Yugoslavia (Bosnia) is described and analysed on the basis of twelve excerpts from the full transcript of the videotaped interaction. The findings show that the untrained ('natural') interpreter clearly fails to maintain a consistent focus on her translatorial role and task and introduces significant shifts in the form as well as the substance of communication. Unaware of the cleaner-interpreter's impact on the interaction, the therapists ultimately lose control over the quality and effectiveness of their professional work.