Cytosine arabinoside was administered to 449 patients with a variety of solid tumors. The phase I study included doses of 0.8–3.5 mg/kg × 10 days, and a small number received 5 mg/kg daily for the first three days of each week. The phase II dosage of 3.5 mg/lg × 10 days was given to 225 evaluable patients. Toxicity was seen at all dosage levels. Anemia occurred in 24–57%, leukopenia in 34–70%, and thrombocytopenia in 8–40% with a tendency for increasing incidence with dosage. Striking rebound of platelet counts after the nadir was seen in 22–37% with counts of over 1,000,000 in 2.7%. Responses occurred in a small number of patients (32/377 evaluable cases). They occurred at all dosage levels clustering in dosages of 2–3.5 mg/kg. The most responsive tumors were breast (8/52), gastrointestinal adenocarcinoma (14/124), and soft tissue and bone sarcomas (3/26). The responses were of short duration and rarely of clinical benefit considering the level of toxicity. Therefore, we found little clinical utility for this drug in the schedule employed and the spectrum of tumors studied.