The emerging interest in essential polyunsaturated fatty acids has resulted in a number of products available in the Danish market. These products contain essential fatty acid oils rich in n-6 and n-3 fatty acids. The quality of these products plays a major role in planning prophylactic and clinical studies. This investigation therefore illustrates the quality and the contents of undesirable oxygenated compounds--peroxides and polymeric/oxidative components. Intake of these undesirable substances can theoretically contribute to the development of cancer. It is therefore of utmost importance to be aware of the quality of the product especially when relatively large doses are employed for a considerable period. Our results show that the oxidation values of the fish oil products are much higher than vegetable oils. Furthermore we observed higher amounts of polymerised/oxidised components in Pikasol although the peroxide value was relatively low. Okholm's fish oil and cod liver oil contain higher amounts of undesirable gadoleic and cetoleic acids. A quality control from the authorities is desirable as the consumer has only the producer's/importer's "assurances" to rely on and these are not always in accordance with the real facts.