["Changes in bile acid (BA) pool, such as the reappearance of typically foetal-type molecular species with a 'flat' structure at the steroid ring, occur during hepatocarcinogenesis, both in humans and rodents. Moreover flat-BAs also appear during rat liver regeneration. These changes can be detected in urine. The aim of the present study was to investigate whether flat-BAs also reappear during human liver regeneration, and whether this change correlates with the magnitude of liver resection.", 'Patients undergoing partial hepatectomy were divided in two groups: major hepatectomy group (> 50% of hepatic tissue resection, n = 17) and minor hepatectomy group (< 50%, n = 13). BAs were extracted from serum and urine (collected over 24 h) and analysed by gas chromatography-mass spectrometry. Samples were obtained before surgery (day 0) and on the third and seventh days after hepatectomy.', 'In serum, total BAs significantly increased on day seven after hepatectomy, but only a moderate increase in flat-BA concentrations was observed. By contrast, urinary excretion of total as well as flat-BAs significantly increased at day three and day seven after hepatectomy. Moreover, the amount of flat-BAs excreted in urine during the first week after partial hepatectomy correlated with the magnitude of the resection.'] Do foetal 'flat ' bile acids reappear during human liver regeneration after surgery?