Pyrazofurin was administered to 21 patients with solid tumors at a dose of 200 mg/m2 iv weekly, because this dose had been shown to be well-tolerated and pharmacologic effects of a single dose at this level persisted for up to 7 days. An anemia consistent with a disturbance in rbc production was seen in most patients. Other toxic effects included stomatitis, rash, and myelosuppression. No complete or partial responses were noted, but two patients with alveolar cell carcinoma of the lung each had stable disease for 12 months. Most of the patients in this study tolerated the weekly dosage schedule well with only minimal myelosuppression, suggesting that this agent and schedule might be acceptable for use in combination chemotherapy. Several theoretic reasons favor the use of pyrazofurin in this manner. Pyrazofurin should also be evaluated more fully in patients with polycythemia vera, mycosis fungoides, and psoriasis, since other orotidylate decarboxylase inhibitors have been shown to be effective in these diseases.