Despite nine studies reporting the results achieved when treating patients with chronic myeloid leukemia (CML) with interferon (rIFNα) and cytarabine (araC), the optimal doses and schedule for this combination remain to be determined. Results of imatinib mesylate (STI-571) in chronic phase CML are preliminary, thus, trials of rIFNα-2b/araC in CML are of continued interest. We report the results of CALGB study 9013, providing a 10-year follow-up on 88 evaluable previously untreated patients. Cycles of therapy with rIFNα-2b and araC sufficient to cause a decline in either the white blood cell (WBC) count to < 2000/μl or platelets to < 50,000/μl were given. The starting dose of rIFNα-2b was 5 million units (mu)/m2/day subcutaneously (sc) and of araC 10 mg/m2 twice daily sc. Treatment was discontinued when cytopenia occurred and was restarted when both the WBC and platelet counts had recovered. Bone marrow was obtained regularly for morphologic, cytogenetic and molecular studies. Medians at entry included age 48 years, WBC = 89,900/μl and platelets = 345,000/μl. The performance status was 0 or 1 in 88%; splenomegaly was present in 46%. Fifty five (63%) patients had a complete hematologic response and 10 (11%) had a partial hematologic response for an overall response rate of 74%. Median time to best response was 5.3 months. Median survival for all patients from study entry was 81 months, the 5-year survival probability was 65%. When 28 patients were censored at the time of bone marrow transplantation the median survival was 82 months. Grade 3 anorexia, nausea, vomiting and diarrhea developed in 15, 27, 13 and 7%, respectively. Mild to moderate elevations of transaminases occurred in 42%, and were severe in 5%. Sixty-three patients had adequate follow-up cytogenetic studies: 10 had a complete cytogenetic response (CCyR), 23 partial (PCyR, 50-99% normal cells), 20 minor and 10 no response (CALGB criteria). Thus, the CCyR plus PCyR rate among these 63 patients was 52%. Assuming the 25 patients with no cytogenetic follow-up as non-responders, 38% of the 88 patients had at least a PCyR. The median time to CCyR or PCyR was 5.6 months. The median time to best response in these 33 patients was 10.0 months, and median duration of cytogenetic response was 28 months. Cytogenetic responders had significantly longer survival than non-responders (p = 0.01) using a landmark analysis at 18 months. This intermittent schedule of rIFNα-2b/ara-C has a high response rate in patients with CML with acceptable toxicity.