This paper aims at assessing the setpoint tracking performance of PID control loops, under certain constraints such as the processes under control are linear-time invariant and the instrumentations of control loops work properly. The lower bounds of integrated absolute errors (IAEs) are established, based on the widely used internal model control (IMC) principle, from closed-loop responses subject to setpoint changes in the form of step, ramp or other general types. Taking the lower bound as a benchmark, an IMC-IAE-based index is proposed to assess the setpoint tracking performance of PID control loops. Numerical and experimental examples, as well as an industrial case study, are provided to verify the lower bound as the performance benchmark and to illustrate the effectiveness of the proposed performance index. (C) 2011 Elsevier Ltd. All rights reserved.