Adenosine 5'-triphosphate (ATP) has antineoplastic activity in vitro and in murine tumor systems, but there are no data in humans defining its potential use as an antineoplastic agent. We conducted a Phase I study to determine the spectrum of toxicity, maximum safely tolerated dose (MTD), and pharmacokinetics of intravenous ATP. Fourteen men with advanced cancer received 96-hour infusions of ATP once monthly in doses ranging from 50 to 100 pg/kg/minute. Toxicity was assessed by standard National Cancer Institute (NCI) criteria, cardiac function was monitored serially by two-dimensional echocardiography, and whole blood ATP was measured serially in a subset of patients. ATP was generally well tolerated and no significant hematologic toxicity was noted. The dose-limiting toxicity was a cardiopulmonary reaction characterized by chest tightness and dyspnea that resolved within seconds of discontinuing ATP. Dose-limiting cardiopulmonary toxicity occurred in 3 of 3 patients at 100 pg/kg/minute, in 3 of 6 patients at 75 pg/kg/minute, and 4 of 11 patients at 50 pg/kg/minute. Whole blood ATP levels significantly increased with treatment, reaching a steady state by 24 hours and returning to or near baseline by 1 week after treatment. Plateau levels were 63%, 67%, and 11 6% above baseline at 50, 75, and 100 pg/kg/min, respectively. We conclude that prolonged infusions of ATP are feasible with acceptable toxicity and that 50 pg/kg/minute is both the MTD and the most appropriate dose rate for subsequent Phase II testing of 96-hour infusions of ATP in patients with advanced cancer.