The fatty acid (FA) composition of the main plasma lipids was analysed in eight well-nourished, generally healthy Nigerian children aged 14.1 + 7.2 months and in 17 malnourished children (8 marasmus, 9 kwashiorkor) aged 14.6 _+ 3.8 mouths within the first 2 days of admission at the Dept. of Child Health, University of Benin. In comparison to the control group, the malnourished children showed a marked decrease of polyunsaturated FA with low linoleic acid, mainly in sterolesters (STE), and severely reduced linoleic acid metabolites, including arachidonic acid, in all lipid fractions, o)-3-FA were not altered except for a reduction of docosapentaenoic and docosahexaenoic acids in phospholipids. Clearly increased values were found for saturated FA in STE and for the non-essential monoenoic FA in alllipid classes. This pattern indicates the presence of essential fatty acid deficiency in the malnourished children. There was no significant difference between marasmus and kwashiorkor. Eight malnourished children were followed up in the early phase of recovery during hospital treatment 14.0 _+ 3.1 days after obtaining the first sample. Linoleic acid had increased again in STE, but its metabolites were as low or even lower than before. An impaired activity of delta-6-desaturase, the rate limiting enzyme of linoleic acid metabolism, in suggested by elevated substrate-product-ratios of this enzyme in untreated children with protein energy malnutrition and in the early phase of recovery, which may be due to low insulin levels, protein and zinc deficiency. The trientetraen-ratio (20: 3co9/20: 4c06) thus is not a reliable indicator of essential FA status in protein-energy malnutrition.