In 1987, a group of teachers including three translators and two linguists received funds to carry on a one-year project on error analysis of English into Spanish translations. The project was originated after systematically observing that in the translation courses the nature of the errors made by the students varied widely. This paper is a preliminary report of the studies, analyses and discussions carried out by the group in a six-month period. The corpus of the study includes translations from English into Spanish, that is, from a foreign language into the translator’s native tongue. The authors’ research focuses on the definition of the concept of translation error. The authors attempt a classification of errors after analysing a sample of approximately 160 translations made by 20 students from the first and last courses of the program at the Catholic University of Chili.