Under both the ADA (Americans with Disabilities Act) and the FMLA (Family Medical Leave Act), an employer can ask for medical certification of a disability or of the need for leave.
For a disability, be sure that you tailor the request for information to the specific duties of the job (this is a good time to have up-to-date job descriptions, complete with physical requirements). Provide the description and ask if the employee is capable of performing these duties; if they can’t, are there any reasonable accommodations that will allow the employee to perform them? You want only information you can use to find an accommodation for the employee, or to discover that there is no reasonable way to do so; you don’t want extraneous information that you then have to safeguard in the personnel files.
For FMLA leave, use the forms that the government provides; they may be clunky, but it includes all the required information. If the employee returns an incomplete form to you, you are entitled to ask them to provide full information, and they have seven days to do so. At that point you can legally deny leave if you don’t have full information – but it is wiser to make the request again and give them seven more days to get the full information in before you deny the leave. If they can’t fix it on three tries, you have better grounds for turning down their request.
Thanks to Jason Ritchie for a great presentation!