Idarubicin, a new synthetic anthracycline analogue, was administered orally to 34 cats with spontaneous tumors. The maximum tolerated dosage was determined to be 2 mg/cat/d given for 3 consecutive days every 3 weeks. Anorexia and leukopenia were found to be dose limiting in cats receiving the drug at a higher dosage. The most common toxicoses seen at the maximum tolerated dosage were leukopenia, anorexia, and vomiting; however, development of toxicoses was not found to be associated with sex, FeLV test result, tumor type, dosage, age, or weight. Idarubicin (2 mg/cat/d for 3 days, q 3 wks) was used to treat 18 cats with lymphoma in which complete remission had been achieved by administration of other chemotherapeutic agents. Median remission duration for these cats was comparable to that reported for cats treated with other protocols. We concluded that orally administered idarubicin would be useful in the treatment of cats with lymphoma.