Malonaldehyde or malondialdehyde (MA) is an ubiquitous dietary constituent formed by the peroxidation of unsaturated fatty acids and of the food additive, sorbate. The average US citizen consumes between 0.2 and 0.6 microgram of MA each day from fresh and processed meats alone. This figure might have been considerably higher in past decades when meat was packaged and stored under less optimal circumstances. No studies seem to have been performed to determine if free MA is released under physiological conditions, such as in simulated gastric juice. Although pure enzymologically synthesized MA is a weak mutagen for a new Salmonella frameshift tester strain containing a polyadenine sequence, reports of more pronounced mutagenicity and carcinogenicity are very likely erroneous. The impure chemically synthesized MA samples used in the strongly positive tests for genotoxic activity were contaminated with mutagenic methoxy- and ethoxyacroleins that would not be expected to constitute important components of foodstuffs. Thus, the greatest importance of MA and other carbonyls in foodstuffs may be their ability to catalyze nitrosation reactions in the presence of free nitrite and as indicators of the presence of other lipid oxidation products.