A monitoring procedure is proposed to assist engineers in deciding whether a redesign of the controller is necessary. The maximum closed-loop log modulus, L-c,L- max, is considered because it measures the robustness of the control systems and thereby can indicate the appropriateness of controller design with respect to the current operating condition. The proposed method extends previous results on monitoring L-c,L- max for SISO control systems and multiloop control systems to 2 x 2 and 3 x 3 full multivariable control systems. The proposed monitoring procedure identifies L-c,L- max on-line without requiring a priori information of process conditions. This procedure is applied to both linear and nonlinear systems. The results show that the proposed method is efficient and accurate in on-line identification of L-c,L- max.