The American Society for Apheresis (ASFA) Committee on Clinical Applications systematically and critically reviews published information on the use of therapeutic apheresis in clinical practice. On the basis of this review, selected diseases are assigned one of five categories (category I, II, III, IV, and P). The diseases, which were classified as category IV indications, and the rationale for such assignment are reviewed in this article. The diseases assigned to category I, II, III, and newly created category P are discussed in a separate article in this issue.