In this paper we present an automatic tuning methodology for PID controllers for distributed-lag processes. The technique is based on the evaluation of a closed-loop set-point or load disturbance step response and it can be therefore employed with process routine operating data. Further, a performance assessment index is also proposed in order to establish when the performance of a PID controller can be improved by retuning it according to the proposed method. Simulation results show the effectiveness of the approach.