Nineteen patients with acute nonlymphocytic leukemia (ANLL) de novo who had no detectable chromosomal abnormalities received intensive remission induction chemotherapy. After the first chemotherapy cycle, ten of 14 (71%) patients aged 60 years or less entered complete remission (CR) whereas none of five patients aged more than 65 years attained CR. On the other hand, leukemia showed a drug resistance in three (21%) of the former patients and in four (80%) of the latter patients. One patient in each group died during induction. Generally, older ANLL patients have an inferior CR rate after chemotherapy and a poor prognosis. This has been explained by the facts that the risk of induction death increases and that specific chromosomal abnormalities associated with poor prognosis are considerably more frequent in older patients. Our data may indicate, however, that in ANLL with normal karyotype older patients show a low initial response rate because of drug resistance.