This report describes our experience with various techniques for the freezing of platelet-rich plasma, removed from the final product after leukapheresis procedures performed on 14 hematological patients. A total of 194 platelet units were frozen for subsequent autologous transfusion, by the following four methods: (1) 6% dimethyl sulfoxide (DMSO); (2) a combination of 5% DMSO/6% hydroxyethyl starch; (3) 3% glycerol; (4) 5% glycerol/4% glucose. Each technique was evaluated by measuring the percentage of platelet recovery, malondialdehyde (MDA) production, and lactate dehydrogenase release. To investigate the safety and therapeutic effectiveness of the previously frozen platelets, in vivo comparison of four platelet freezing methods was made in 8 thrombocytopenic patients, using corrected platelet increment (CCI), determined at 24 h. Our in vitro results indicate that the cryopreservation with 6% DMSO, without controlled cooling rate, provides significantly (p less than 0.05) greater platelet recovery (75%) as compared to other systems. The decrease of MDA production and the increase in plasma lactate measured after the thawing process was less in the DMSO-frozen units than in the other platelet units. When platelets, cryopreserved by this method, were subsequently transfused into patients, a significantly better CCI (greater than 5,000/microliters) was obtained. In our series, 6 patients were entirely supported with frozen autologous platelets. It appears from this study that a better understanding of the physical and biochemical events occurring during the freezing process will improve platelet cryopreservation, allowing a more systematic use of frozen platelets in the support of thrombocytopenic patients.