The study purpose is to evaluate the clinical response to multidose administration of anti-Tac monoclonal antibody conjugated with 10 mCi 90Y in patients with Tac-expressing adult T-cell leukemia (ATL). This study represents an extension of phase I trial (Clinical Project #90-C-0043, FDA IND #3469) which permitted the administration of 5 mCi, 10 mCi, and 15mCi 90Y-anti-Tac to patients with ATL. We propose to administer 90Y-anti-Tac (10 mCi doses) to patients with Tac-expressing ATL who are over 18 years of age and who fill the patient eligibility criteria.