Serum and erythrocyte levels of the polyamines spermine, spermidine and putrescine, as well as ornithine decarboxylase in erythrocytes, were studied in patients with different neoplasms (breast, lung and colon cancer) and in those with a nonmalignant proliferative disease (familial polyposis). The blood levels of polyamines and the spermine/putrescine ratio were significantly higher in all tumors and in nonmalignant colon polyposis. In erythrocyte ornithine decarboxylase activity, spermine and spermidine levels, as well as spermidine/putrescine and spermine/putrescine ratios showed a significant decrease after surgery and chemotherapy. Our data suggest that high levels of blood polyamines and erythrocyte ornithine decarboxylase activity are related to cell proliferation and cancer treatment, but that levels of polyamines in serum and erythrocytes are still significantly high after cancer treatment and are similar to those in polyposis disease. Polyamines are related to nuclear activity during differentiation; therefore, the altered turnover of polyamines could be a sign of abnormal nuclear function. Since polyamines stimulate protooncogene expression, their high levels could be considered an important cofactor in malignant cell transformation.