The increased risk of neoplasia following cytotoxic therapy for both malignant and nonmalignant disease is well known. Sister chromatid exchange (SCE) frequencies in patients receiving such chemotherapy are often elevated, and the persistence of high levels after treatment may provide an indicator for susceptibility to secondary neoplasia. The cytostatic drug razoxane has been used for the treatment of psoriasis, acute myeloid leukemia (AML), and colorectal carcinoma. Prolonged use of this drug, however, has been associated with the subsequent development of AML and, up to November 1987, 16 cases of acute leukemia following razoxane treatment have been reported. We report the SCE frequencies for 34 patients with colorectal carcinoma who were receiving or had previously been treated with razoxane. Our results show no significant increase in SCE levels in the razoxane group compared with either normal controls or untreated patients.