The Food and Drug Administration has recommended that blood donated for transfusion in the United States be screened for antibodies to human T lymphotropic virus type I. Since seropositive donors will be referred to health care providers for advice, physicians should become familiar with HTLV-I, its modes of transmission and its disease associations. According to current knowledge, seropositive persons should neither donate blood for transfusion nor share needles. Women should be advised against breast feeding. Advice about sexual activity is more difficult to formulate and must be individualized.