We report two patients who experienced severe reversible encephalopathy following infusion of peripheral blood stem cells cryopreserved in 10% dimethylsulfoxide (DMSO). In one patient, reduction of DMSO level with plasmapheresis resulted in marked improvement in encephalopathy. Infusion of large volumes of cryopreserved stem cells may result in a significant toxic reaction. Plasmapheresis may be a treatment option for patients with significant toxicity related to DMSO.