We analyzed multidrug-resistance gene (mdr1 gene) expression in a patient with undifferentiated sarcoma of the liver using the cloned cDNA for the mdr1 gene. Tissue samples were available at the time of initial diagnosis and of two intracranial relapses after chemotherapy with a regimen including doxorubicin and teniposide. The level of mdr1 gene expression was increased sevenfold in the intracranial tumor at the time of first relapse and 11-fold at the second relapse. This case may be an example of acquired multidrug resistance associated with overexpression of the mdr1 gene.