Interleukin (IL)-21 has been reported to both stimulate cell growth and promote survival in benign lymphoid cells and several types of hematopoietic neoplasms. It induces JAK3/STAT3 signaling, a biologically important cellular pathway activated in most cases of anaplastic lymphoma kinase (ALK)-expressing anaplastic large cell lymphoma (ALK⁺ALCL). Therefore, we hypothesize that IL-21 may contribute to JAK3/STAT3 activation and cell growth in ALK⁺ALCL. By reverse transcription-PCR, we found consistent expression of IL-21 receptor (IL-21R) in all ALK⁺ALCL cell lines and frozen tumors examined. IL-21 was also consistently expressed in ALK⁺ALCL tumors, although its mRNA was detectable in only one of three cell lines tested. By immunohistochemistry, we examined 10 paraffin-embedded ALK⁺ALCL tumors; all cases were positive for both IL-21 and IL-21R in these neoplastic cells. IL-21 signaling is biologically significant in ALK⁺ALCL since the addition of recombinant IL-21 enhanced the activation of JAK3/STAT3 and significantly increased cell growth in ALK⁺ALCL cell lines. However, small interfering RNA down-regulation of IL-21R significantly decreased both STAT3 activation and cell growth. IL-21R expression is not linked to nucleophosmin-ALK since forced expression of nucleophosmin-ALK and small interfering RNA down-regulation of nucleophosmin-ALK did not significantly change the expression of either IL-21R or IL-21. Our findings thus support the enhancement of JAK3/STAT3 activation and cell growth in ALK⁺ALCL via IL-21 signaling. These results further support the concept that constitutive activation of STAT3 in these tumors is multifactorial.