BACKGROUND The endogenous conversion of linoleic acid into long-chain polyunsaturated fatty acids is of potential importance for meeting substrate requirements, particularly in young infants. After application of [13C]-linoleic acid, we estimated its conversion to dihomo-gamma-linolenic and arachidonic acids from only two blood samples. DESIGN Oral tracer doses were given to five healthy adults as a single bolus. In four subjects the tracer was given in nine equal portions over 3 days. Concentration and 13C content of fatty acids from serum phospholipids were analysed by gas chromatography combustion isotope ratio-mass spectrometry. Areas under the tracer-concentration curves were calculated, and fractional transfer and turnover rates estimated from compartmental models. RESULTS The median fractional turnover of linoleic acid was 93.7% per day (interquartile range 25.3) in the bolus group and 80. 0% per day (6.3) in the fraction group (NS). Fractional conversion of linoleic to dihomo-gamma-linolenic acid was 1.5% (0.9) vs. 2.1% (0.7) (bolus vs. fraction, P < 0.05), and fractional conversion of linoleic to arachidonic acid was 0.3% (0.3) vs. 0.6% (0.3) (bolus vs. fraction, NS). In the fraction group conversion was significantly higher based on areas under the curve. The ratio of tracer concentration in conversion products to linoleic acid 48 h after dosing correlated very well (r >/= 0.94, P < 0.05) with the ratio of areas under the curve. CONCLUSIONS Using areas under the curve overestimates the conversion, because different residence times are not considered. Estimation of conversion intensity appears possible with only one blood sample obtained after tracer application.