We investigated the correlation between the detection of clonal rearrangement of the T-cell antigen receptor gene (TCRR) in lymph node tissue with histopathologic lymph node classification in 33 patients with mycosis fungoides with and without the Sezary Syndrome. We analyzed DNA extracted from lymph nodes that were histologically uninvolved (LN1–2), dermatopathic nodes with clusters of atypical cells (LN3), and nodes effaced with lymphoma (LN4) and found TCRR in none of five LN1–2 nodes, 8 of 17 LN3 nodes, and 10 of 11 LN4 nodes. Further, the detection of TCRR correlated with presence of palpable adenopathy (P2 less than .0001) and was associated with a worse survival (P2 = .0024). Within the subgroup of patients with LN3 nodes, there was a trend (P2 = .14) toward inferior survival if nodes were involved by TCRR, irrespective of extent of skin disease. We conclude that detection of TCRR in nodes from mycosis fungoides patients is an objective and reliable means of assessing tumor infiltration of lymph node and is associated with an inferior survival.