Erythrodermic cutaneous T-cell lymphoma (CTCL) includes patients with erythrodermic mycosis fungoides who may or may not exhibit blood involvement and Sézary syndrome and in whom hematological involvement is, by definition, present at diagnosis. These patients were stratified into 5 hematologic stages (H0-H4) by measuring blood tumor burden, and these data were correlated with survival. The study identified 57 patients: 3 had no evidence of hematologic involvement (H0), 8 had a peripheral blood T-cell clone detected by polymerase chain reaction (PCR) analysis of the T-cell receptor gene and less than 5% Sézary cells on peripheral blood smear (H1), and 14 had either a T-cell clone detected by Southern blot analysis or PCR positivity with more than 5% circulating Sézary cells (H2). Twenty-four patients had absolute Sézary counts of more than 1 x 10(9) cells per liter (H3), and 8 patients had counts in excess of 10 x 10(9) cells per liter (H4). The disease-specific death rate was higher with increasing hematologic stage, after correcting for age at diagnosis. A univariate analysis of 30 patients with defined lymph node stage found hematologic stage (P =.045) and lymph node stage (P =.013) but not age (P =.136) to be poor prognostic indicators of survival. Multivariate analysis identified only lymph node stage to be prognostically important, although likelihood ratio tests indicated that hematologic stage provides additional information (P =.035). Increasing tumor burden in blood and lymph nodes of patients with erythrodermic CTCL was associated with a worse prognosis. The data imply that a hematologic staging system could complement existing tumor-node-metastasis staging criteria in erythrodermic CTCL.