This paper describes an application of the H-infinity optimal control theory to the design of an engine idle speed electronic regulator. This technique, working directly on multi-input multi-output systems, allows to achieve optimal performances, good disturbances rejection and robustness to parametric variation of the plant. Compared to the current heuristic techniques for the tuning of the control parameters, the following procedure offers the opportunities to become completely automatic, reducing in this way time and efforts. The experimental results, finally, are better than those obtained by conventional P.I.D. regulators and equivalent of an optimal L.Q.I, ones.