Umbilical cord blood transplantation using nonmyeloablative conditioning is currently considered by many as a valid potential alternative for any patient who requires an unrelated donor allograft and who is without a suitably matched and readily available volunteer. Dimethyl sulfoxide (DMSO) has been used for years as a cryoprotectant agent; it acts by penetrating the cell and binding water molecules and it has been described as harmless for the individual who receives it in limited amounts. In this paper, we describe 3 cases of DMSO-induced toxicities and briefly review the most common adverse reactions of the DMSO when used as a cryopreservation agent for the long-term storage of cord blood cells. Two of the 3 cases had a dismal prognosis. A brief review of the literature is presented.