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The primary objective of this phase II study was to evaluate the efficacy of rituximab in the management of adult patients with physician-diagnosed presumed thrombotic thrombocytopenic purpura (TTP); relapsed or refractory. We conducted a multicentre study in four Canadian hospital-based apheresis units. Forty patients with presumed TTP (20 refractory and 20 relapsing) were sequentially enrolled and all received rituximab in a standardized manner. A complete response was documented in 14 of 19 refractory patients by week 8 and 15/16 were alive and in remission at 52 weeks (one patient was lost to follow-up, one was a non-responder, and three died). Among relapsing patients, 16/18 had a complete response at week 8 and 18/18 at week 52 (one patient lost to follow-up and one withdrew). At 1 year, all relapsing and 85% of refractory patients survived. Of 38/40 patients who had ADMATS13 testing at study entry, 13/19 refractory and 10/19 relapsing patients had ADAMTS13 10% (typical TTP); whereas 6/19 refractory and 9/19 relapsing cases had ADAMTS13 10% (other thrombotic microangiopathy; TMA). Refractory-typical TTP in contrast to refractory-other TMA and all relapsing patients treated with plasma exchange and rituximab, were less likely to be responsive and more likely to die or relapse.