An observational study was conducted to describe the presence of plagiarism in medical thesis in 2008 performed at a public university in Peru. Search for plagiarism in 33 thesis introductions using a Google search algorithm, characterizes of the study type and we search in electronic form if the thesis mentor have published articles in scientific journals. We found evidence of plagiarism in 27/33 introductions, 37.3% (171/479) of all the paragraphs analyzed had some degree of plagiarism, literal plagiarism was the most frequent (20/27) and journals were the most common sources of plagiarism (19/27). The characteristics of the studies were observational (32/33), cross-sectional (30/33), descriptive (25/33) and retrospective (19/33). None of the authors had published in a scientific journal, and only nine of his tutors of them had at least one publication. No association was found between the characteristics of the thesis and the presence of plagiarism. In conclusion, we found a high frequency of plagiarism in theses analyzed. Is responsibility of medical schools take the necessary actions to detect and avoid plagiarism among their students. / Revisión por pares.