Regressing atypical histiocytosis. Aberrant prothymocyte differentiation, T-cell receptor gene rearrangements, and nodal involvement. Two cases of regressing atypical histiocytosis were studied. New findings reported include the presence of CD1 epitope on large atypical histiocytoid cells and apparent early lymph node involvement. Because regressing atypical histiocytosis may demonstrate aberrant thymic differentiation antigen expression and T-cell receptor gene rearrangements, initial lymph node involvement, and eventuation in conventional lymphoma in some cases, the alternative term regressing-phase anaplastic lymphoma is proposed as a more accurate designation for this entity. 