Previous studies have shown that phenylketonuric patients display a deficiency in long-chain polyunsaturated fatty acids. A study has now been performed on 13 cases (5 with methylmalonic acidaemia and 8 with urea cycle disorders) whose dietary treatment also implies a limitation in protein-rich food. Plasma and red-cell phospholipid fatty acid profiles were studied. The most relevant results were a lower percentage of docosahexaenoic acid in plasma and red-cell phospholipids (0.91% +/- 0.53% vs 2.88% +/- 1.17% and 2.07% +/- 0.92% vs 3.62% +/- 1.01% (p < 0.001)) and a lower percentage of arachidonic acid in plasma (5.22% +/- 2.02% vs 8.3% +/- 2.11% (p < 0.001)). A long-chain polyunsaturated acid deficiency has also been confirmed in this group of metabolic patients and a dietary supplement is recommended since this population is subject to a special risk factor with regard to adequate psychomotor development. By extrapolating these data to the general population, the possibility can be inferred that long-chain polyunsaturated fatty acids are semi-essential in infant nutrition far beyond the breast-feeding period.