The decision to make a claim for priority used to be a no-brainer. However, a change to the patent laws in 1995 altered the way in which a patent term was calculated from 17 years from the date of issue to 20 years from the filing date of the earliest application. Accordingly, a priority claim may decrease the potential term of your patent. However, sometimes a claim for priority may be necessary to avoid prior art.
A patentee may also claim priority with a foreign application as long as there is a common inventive entity and the patent is filed within one year of the foreign application (six months for design patents). The claim for priority need not be made at the time of filing. In fact, a patentee may even make a claim for priority after the issuance of the patent in a reissue application.