The results prompted researchers to warn tutors to be on the look out for "suicidal thoughts", with hospitals more aware of the stresses trainee doctors are under.
Scientists found that almost four in 10 young medics in one medical school showed symptoms of depression, including insomnia, social withdrawal and suicidal thoughts.
The signs of the illness intensified as the trainee doctors prepared to enter hospitals for the first time, the study found.
Researchers believe that a fear of failure and worries over performing intimate physical examinations on patients, as well as long hours and stress, were to blame.
Mental health charities warn that around one in five people will suffer from depression at some point in their lives.
The British Medical Association says that more than 50 per cent of medical students admit that they have sought help for depression or other mental health problems.
Researchers found that female medical students were more likely to show symptoms of depression than their male classmates.
But those who had a parent who was a doctor were less likely to suffer signs of the condition, the study found, suggesting that they were more aware of the pressures of the profession before they started medical school.
The study, the findings of which are published in the journal BMC Medical Education, looked at 481 students at a private university.
The results showed there was a "high prevalence" towards depressive symptoms among young doctors.
Frequently "students fear they 'know nothing' and are insecure about the physical examination of other people", it warns.
Most common symptoms included sadness, dissatisfaction, spontaneous crying, irritability and withdrawal, the study found.
But students also suffered increased tiredness, weight loss and a lack of interest in sex.
The authors warn that universities and hospitals should be more aware of the stresses that trainee doctors are under and look for warning signs.
Sergio Baldassin, from the ABC Regional Medical School in Brazil, who led the study, said: "This is probably the period when professors and educators should try to be most aware of suicidal thoughts and risk in their students."
