Over 22 million US residents are limited English proficient. Hospitals often call upon untrained persons to interpret. There is a dearth of information on errors in medical interpreting and their impact upon cancer education. We conducted an experimental study of standardized medical interpreting training on interpreting errors in the cancer encounter, by comparing trained and untrained interpreters, using identical content. Nine interpreted cancer encounters with identical scripts were recorded and transcribed. Using an "Error Analysis Tool," a bilingual linguist and two bilingual medical providers scored the transcripts for interpreting errors made, including their potential clinical severity. Trained interpreters were 70% less likely to have clinical errors than untrained ones. The likelihood of medical error increased with the length of the concept and decreased with the precision of vocabulary. It is important to train medical interpreters and to ensure their availability in cancer education encounters to minimize the risk for errors.