OBJECTIVES: I. Evaluate the safety, maximum tolerated dose, adverse effects, and toxicities of cordycepin, given 1 hour following a fixed dose of the adenosine deaminase inhibitor pentostatin, in patients with refractory TdT positive leukemia. II. Determine the single and multiple dose pharmacokinetics of cordycepin given 1 hour following a fixed dose of pentostatin. III. Characterize selected whole blood and blast cell metabolic parameters serially in relation to cordycepin/pentostatin administration. IV. Measure and quantify any clinical responses in refractory TdT positive leukemia patients following cordycepin/pentostatin administration.