Although cytotoxic chemotherapy for human cancer has been reported to induce alterations in intestinal permeability, its effects on the absorptive process are still controversial. We have studied mediated and nonmediated absorption in 10 patients with metastatic breast cancer before and after treatment with Adriamycin by the use of specific test sugars given orally and their subsequent urinary recovery, as measured by chromatography. Mediated absorption was investigated by the use of D-xylose and 3-O-methylglucose, while lactulose and L-rhamnose were used to study nonmediated permeation. Lactulose is considered a marker of unmediated paracellular (tight junction) permeation, while L-rhamnose explores passage across cell membranes. The test was performed on patients before and on the second and the eighth days after Adriamycin administration, and only once in 22 age-matched healthy women. Under basal conditions, as well as 2 and 8 days after chemotherapy, D-xylose and 3-O-methylglucose absorption was 35% lower in patients than in controls (P less than 0.001). Lactulose absorption was significantly higher in patients than in controls under basal conditions (P less than 0.001); it reached levels three times higher the second day after chemotherapy, and returned to basal levels by the eighth day. The data suggest an early reversible effect of Adriamycin on cellular tight junctions with resulting increased permeabilization. This effect seems of a toxic nature rather than due to increased cell loss. It is interesting that both nonmediated absorption and mediated absorption were already altered before chemotherapy in cancer patients, suggesting a preexisting functional damage of the intestine. The significance of this alteration as a potential mechanism of cancer cachexia is discussed.