The invention relates to a method for estimating the interference level in a cellular radio system comprising in each cell a base station (100) in each cell, and a number of subscriber terminals (108 to 112) communicating with the base station. The terminals measure power levels from those frequencies which are on the list that is maintained by the terminal and comprised those base stations and frequencies to which the terminal can perform a handover. In the solution of the invention the list also comprises frequencies whose power level information is used for the frequency planning of the system. Frequency planning can thus be implemented without any separate means.