The United States is experiencing one of its largest migratory waves, so health providers are caring for many patients who do not speak English. Bilingual nurses who have not been trained as medical interpreters frequently translate for these patients. To examine the accuracy of medical interpretations provided by nurses untrained in medical interpreting, we conducted a qualitative, cross-sectional study at a multi-ethnic, university-affiliated primary care clinic in southern California. Medical encounters of 21 Spanish-speaking patients who required a nurse-interpreter to communicate with their physicians were videorecorded. Encounters were transcribed by blinded research assistants. Transcriptions were translated and analyzed for types of interpretive errors and processes that promoted the occurrence of errors. In successful interpretations where misunderstandings did not develop, nurse-interpreters translated the patient's comments as completely as could be remembered and allowed the physician to extract the clinically-relevant information. In such cases, the physician periodically summarized his/her perception of the problem for back-translation and verification or correction by the patient. On the other hand, approximately one-half of the encounters had serious miscommunication problems that affected either the physician's understanding of the symptoms or the credibility of the patient's concerns. Interpretations that contained errors that led to misunderstandings occurred in the presence of one or more of the following processes: (1) physicians resisted reconceptualizing the problem when contradictory information was mentioned; (2) nurses provided information congruent with clinical expectations but not congruent with patients' comments; (3) nurses slanted the interpretations, reflecting unfavorably on patients and undermining patients' credibility; and (4) patients explained the symptoms using a cultural metaphor that was not compatible with Western clinical nosology. We conclude that errors occur frequently in interpretations provided by untrained nurse-interpreters during cross-language encounters, so complaints of many non-English-speaking patients may be misunderstood by their physicians.