Summary:Fifteen cancer patients were studied following repeated courses of doxorubicin (12‐44 mg/m2) (together with other anticancer agents) to consider the possibility of enhanced metabolism as a cause of the previously reported reduction in doxorubicin plasma concentrations with repeated courses. Plasma doxorubicin and doxorubicinol concentrations were measured by a modified high‐performance liquid chromatography/fluorescence method. The results presented confirmed the significant decline in doxorubicin plasma concentration‐dose ratios measured 3 h after the 1‐h infusion. Although the degree of this reduction varied markedly between patients, it was shown not to be associated with a rise in the plasma concentration‐dose ratio of the major metabolite doxorubicinol or with altered renal and/or hepatic function, which may have influenced disposition. Alternative mechanisms that might explain the reduction in doxorubicin concentrations, such as increased doxorubicin clearance or volume of distribution, were not considered in the present study.