The dietary origin of the weak oestrogen equol (7-hydroxy-3-(4'-hydroxyphenyl)-chroman) present in human urine has been investigated using gas chromatography-mass spectrometry. Feeding experiments with different food constituents and monitoring the urinary excretion of equol revealed that soya food yields more than 0.1 mg urinary equol/g flour ingested. From this source the glucoside of daidzein (4',7-dihydroxyisoflavone) has been isolated and identified as a precursor of equol. Both equol and daidzein were characterized as monoglucuronide conjugates in human urine and the concentration of urinary equol exceeded the concentrations of the classical oestrogens by 100- to 1000-fold after ingestion of a single meal containing soya protein. The potential biological significance of this result is discussed.