STUDY OBJECTIVE--The aim was to examine the hypothesis that hydrogenated fats, particularly those obtained from marine oils, may present a health hazard. DESIGN--Storage fat specimens obtained at necropsy were collected from several areas in England and Wales during 1975-1978. Cases (n = 136 samples) consisted of males dying of ischaemic heart disease, male deaths from unrelated causes acting as controls (n = 95 samples). The fatty acid compositions of the specimens were determined, and analysis included those acids--16:1 trans and "higher" C-20 plus C-22 (H)--highly characteristic of partially hydrogenated marine oils. MEASUREMENTS AND MAIN RESULTS--The case samples, which had been shown to be the richer in 16:1 trans (p less than 0.005), were now found to have a significantly higher value of the ratio 16:1 trans to H (p less than 0.002), arising from consumption of differing hydrogenated marine oil types. CONCLUSIONS--It is concluded that the cases had consumed a greater amount (p less than 0.001) of hydrogenated marine oils of a certain type, ie, that manufactured from certain highly unsaturated raw oils. The process of partial hydrogenation results in conversion to a product containing large amounts of polyunsaturated acids (PUFA) which are no longer in the natural all-cis methylene interrupted configuration. Such isomeric PUFA may obstruct or compete with utilisation of natural PUFA. It is further concluded that the case excess did not rise from medical advice favouring margarine or from any difference in social class status, but rather from fortuitous selection of margarine brand.