The literature indicates that the incidence of adverse effects occurring in hospitals is 3,7%, of which 27,6% would be the result of an error, 2,6% caused permanent incapacity and 13% were lethal, with an estimated annual cost 20 to 30 billions US dollars. We have designed a four-phase study, to initiate the search for errors and their systematic prevention in our department. and hereby present the final results of the firs phase. 37% of medical records presented one or more errors, with a total of 59 identified errors (average of 1.6 errors per chart). In 65 records (charts) in which there were no identifiable errors, the most frequent diagnosis was acute appendicitis (35%). Diagnosis in charts that exhibited errors were more varied and correspond to low-incidence entities. Regarding the type of error, the majority fell in the non-classifiable category: “registry error” (37%), followed by diagnostic errors (29%: 17/59), therapeutic errors (22%; 13/59) and preventive errors (12%: 7/59). There is an inverse relationship between the frequency and the familiarity of doctors with certain types of pathology and the occurrence of errors, and the committed error may lead to the occurrence of ulterior errors. There are specific associations between the type of error and the location where it occurs. There are specific associations among the different types of errors, which had not been described before.