A phase I study of tricyclic nucleoside phosphate was conducted in 20 adults with advanced cancer. Tricyclic nucleoside phosphate was given as an iv infusion over 15 minutes once every 3 weeks; the doses ranged from 25 to 350 mg/m2. Beginning at a dose of 250 mg/m2, hyperglycemia and elevation of hepatocellular enzymes were observed; at a dose of 350 mg/m2, two patients developed irreversible liver damage. Patients at all dose levels experienced reduction in serum phosphorus; reduction of serum calcium was noted only with the two highest doses. Nausea and vomiting occurred occasionally. Myelosuppression was not a prominent toxic effect. No major therapeutic responses were noted. Further clinical trials employing this schedule are probably not warranted.