This paper reports the results of a work program conducted in Italy during 1975 and 1976 by the C.U.N.A. Octane Requirement Working Group aimed at establishing the minimum Octane Number in Primary Reference Fuels (PRF) corresponding to a knock intensity which does not induce mechanical engine damage; this O.N. level is indicated as «Real Octane Requirement».The experimental work, conducted by the ESSO, FIAT, MOBIL and SNAMPROGETTI-AGIP Research Laboratories, consisted in running endurance bench tests at 4,000 and 5,000 r.p.m., full load, for 50 hrs (sometimes 100 hrs). The test fuel used was a gasoline having such an Octane rating that would cause the test engine to knock with an intensity equal to that induced by a PRF having an octane rating 2, 3, or 4 units lower than the PRF causing audible trace knock.The ΔO.N. = -2, -3, or -4 was defined as «Octane deficiency» relative to the O.R. determined as trace knock according to the standard procedure.The bench tests have been complemented with field tests run with cars equipped with an air brake and operated at constant speed and full load.On the whole, 37 tests have been performed using the most critical Italian engines; 29 tests were run on the bench and 8 on the road.These tests followed a study aimed at establishing the relationship between engine parameters and knock intensity.