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Denileukin diftitox (DD) is approved for treatment of CD-25 expressing cutaneous T-cell lymphomas (CTCL). Initial studies of DD demonstrated responses in patients with B-cell non-Hodgkin lymphoma (NHL). This phase II trial evaluated response rate (RR) and tolerability of DD in this population. Patients were stratified into two arms: those with NHL expressing > or =20% IL-2R (IL-2R+) or <20% IL-2R (IL-2R-). DD was dosed at 18 microg/kg/day for 5 days every 21 days. Corticosteroid pre-medication was not allowed. Thirty-five patients of a planned 77 accrued due to closure for slow accrual. This report is on 29 patients (18 males) with indolent B-cell NHL (11 IL-2R+ and 18 IL-2R-). Histologic subtypes included small lymphocytic (SLL) (8 patients) and follicular grade I/II lymphoma (21 patients). Patients received a median of three prior regimens, including rituximab in 76%. Three partial responses were observed (RR 10%). The RR for the IL-2R- and IL-2R+ patients was 11% and 9%, respectively. Of 8 patients with SLL, 2 responded. Toxicities were generally grade I - II and transient but 1 patient experienced a fatal thrombo-embolism. Therapy with DD is tolerable and modest efficacy was observed in SLL subtype. Measured IL-2R status did not correlate with efficacy.