An explanation of why medical opinions of workplace exposures and diseases are more reliable if based upon epidemiological studies than on the opinions of individual doctors. Pre-placement medical examinations are discussed. Such examinations have been limited by the ADA: they can be performed only if offered to all applicants, and can be a basis for denial of employment only if the results indicate the candidate is incapable of performing the job or his/her health is at imminent and substantial risk of harm.