Hematopoietic stem cells (HSC) can be stored for prolonged periods at cryogenic temperatures. The techniques currently used were derived from the initial report in 1949 of cryopreservation of bovine sperm in glycerol. The addition of this penetrating cryoprotectant protected the cells from the injury associated with ice formation. Current cryopreservation techniques (with minor variations) suspend cells in an aqueous solution of salts, protein, and one or more cryoprotectants. Cells are frozen at slow rates and stored generally below -120 degrees C in mechanical freezers or nitrogen refrigerators. That these techniques are successful in maintaining HSC viability is evident from the engraftment of these cells in patients treated with marrow-lethal conditioning regimens. However, issues such as the composition of the cryoprotectant solution, cell concentration during freezing, cryoprotectant toxicity, and storage temperatures have not been adequately studied, primarily because of a lack of appropriate assays for HSC cryosurvival. HSC cryobiology will become an increasingly important subject as new HSC collection and processing techniques are developed. Improved cryosurvival of HSC using modified cryoprotectant solutions may improve engraftment kinetics and decrease the cost and morbidity of autologous transplantation.