Skin biopsies were collected from 15 patients with cutaneous tumors morphologically similar to Ki-1+ anaplastic large cell (ALC) lymphoma of the lymph nodes, including Ki-1+ ALC lymphoma of childhood. The histology and immunophenotype of these cutaneous tumors are reported and follow-up data on the patients are given. The tumorous infiltrates were composed of large, sometimes very bizarre cells with one nucleolus or multiple nucleoli. All tumor cells expressed the lymphoid cell activation antigen Ki-1 (CD30) in conjunction with CD25 and the beta-chain of the T cell receptor. In 11 patients, Ki-1+ cutaneous tumors developed primarily in the skin. In nine patients, these were restricted to the skin in follow-up periods ranging from 3 months to 6 years. Two patients developed lymph node involvement after 2 months and 2 years, indicating the spreading potential of these cutaneous tumors. Morphologic and immunophenotypical identity of the atypical cells found in primary cutaneous ALC lymphoma, in regressing atypical histiocytosis (RAH), and in lymphomatoid papulosis (LyP) of type A, together with the protracted clinical course in all three conditions, suggests that primary cutaneous ALC lymphoma, RAH, and LyP of type A represent clinical variants of the same lymphoma entity. Secondary development of Ki-1+ ALC skin tumors was observed in two patients with cutaneous T cell lymphomas of mycosis fungoides type. These secondary Ki-1+ ALC lymphomas of the skin showed rapid systemic progression similar to the primary lymphonodal Ki-1+ ALC lymphomas. Concomitant or subsequent occurrence of Ki-1+ ALC tumors in cutaneous T cell lymphomas thus may be a bad prognostic sign.