Registration/deregistration is required in a PCS network when a portable moves between registration areas. Several schemes were proposed to deregister a portable after it moves out of a registration area. A simple scheme called implicit deregistration totally eliminates network traffic due to deregistration. However, this scheme may delete valid registration records. Thus, the size of a registration database must be sufficiently large to ensure low probability that a valid registration record is deleted. This paper describes an analytic model to determine the size k of the registration database for an RA in the implicit deregistration scheme. If the expected number of portables in an RA is N, then our study indicates that good performance can be achieved if k ≈ 5N.