<jats:p><jats:bold>Aims </jats:bold> To examine diurnal variation in biliary excretion of flomoxef.</jats:p><jats:p><jats:bold>Methods </jats:bold> Flomoxef (1 g) was injected intravenously in eight patients with percutaneous transhepatic cholangiography with drainage at 09.00 h and 21.00 h by a cross‐over design with a 36 h washout period. Drained biliary fluid was collected for 6 h after each dosing. These patients still had mild to moderate hepatic dysfunction.</jats:p><jats:p><jats:bold>Results </jats:bold> Bile flow and bile acid excretion for 6 h after dosing did not differ significantly between the 09.00 h and 21.00 h treatments. The maximum concentration of biliary flomoxef was significantly greater and its total excretion for 6 h tended to be greater after the 21.00 h dose [maximum concentration (µg ml<jats:sup>−1</jats:sup>): 34.2 ± 29.9 (09.00 h dose) <jats:italic>vs</jats:italic> 43.5 ± 28.3 (21.00 h dose) (95% confidence interval for difference: 2.6∼15.9, <jats:italic>P</jats:italic> = 0.013); total excretion (mg 6 h<jats:sup>−1</jats:sup>): 1.4 ± 1.3 (09.00 h dose) <jats:italic>vs</jats:italic> 1.6 ± 1.2 (21.00 h dose) (95% confidence interval for difference: −26.8, 313.7, <jats:italic>P</jats:italic> = 0.087)]. The period that biliary flomoxef remained above the minimal inhibitory concentration did not differ significantly between the two treatment times.</jats:p><jats:p><jats:bold>Conclusions </jats:bold> These results suggest that biliary excretion of flomoxef shows diurnal variation. However, as the difference was relatively small, flomoxef could be given at any time of day without any dosage adjustments.</jats:p>