We present the results of an open-label clinical trial and the clinical use of alemtuzumab in 19 heavily pretreated patients with advanced erythrodermic cutaneous T-cell lymphomas (CTCL) (erythrodermic mycosis fungoides and Sézary syndrome). Ten patients received alemtuzumab intravenously using an escalating dose regimen with a final dose of 30 mg three times weekly for 4 weeks followed by subcutaneous administration for 8 weeks. Nine patients were treated with only the SQ or IV dosing. The overall response rate was 84%, with 9 (47%) complete and 7 (37%) partial remissions. The median follow-up was 24 months (range, 6 to 62+ months). Median overall survival was 41 months whereas median progression free survival was 6 months. Minimal residual disease by T-cell gene rearrangement studies was detected in 11 patients who achieved complete response and partial response. Toxicities included myelosuppression and infections; however, the majority of side effects were of Grade 2 in severity and transient. One patient was diagnosed with a concurrent lymphoma (mantle cell lymphoma) 6 months after completing alemtuzumab therapy. Alemtuzumab is particularly effective in patients with erythrodermic CTCL with acceptable toxicities. Combined strategies with alemtuzumab may achieve molecular remissions with longer response durations.