Introduction Illegal practice of medicine by medical students is a worldwide problem. In Brazil, information about this issue is scarce.

Objective To describe the perception of illegal practice of medicine by medical students.

Methods A cross-sectional study in a stratified random sample of 130 medical students in the 6th to 12th semesters from a private faculty of medicine in Salvador, State of Bahia, Brazil, from September to October 2011. Students responded to a standardised questionnaire about the illegal practice of medicine by medical students.

Results Knowing medical students who practised medical activities without supervision was reported by 86% of the respondents, and 93.8% had heard about someone who performed such practices. Medical specialties most often associated with illegal practice were general medicine (78.8%) and occupational health (55.9%). Illegal practice of medicine was more common in peripheral cities/towns (83.9%) than in the State capital, Salvador City (52.4%). Only 10.5% of illegal activities were reported to the authorities. Unsupervised medical practice was more often reported in the 8th–9th semester (56.8%) and 10th–11th semester (54.4%) of medical school.

Conclusions Illegal practice of medicine was commonly reported by the medical students questioned. The high frequency of reported illegal practice for financial reasons highlights the need for greater availability of paid internships for medical students. Educational institutions represent the social control responsible for supervising the activities of academics.