This study concerns the effects of a slow freezing and thawing protocol using dimethylsulfoxide (DMSO) as a cryoprotectant on the survival and viability of 319 supernumerary human embryos produced after in vitro fertilization. One hundred twenty-one transfers were performed in a natural cycle and 18 pregnancies were achieved (15%), from which 14 were ongoing (12%). Overall, 52% of the thawed embryos retained at least 50% of their initial blastomeres intact after thawing, and were replaced. Survival was strongly correlated to prefreezing embryonic quality, as 78% of type I embryos, 55% of type II, 40% of type III, and none of type IV could be transferred. Implantations were obtained from type I embryos (21% per embryo replaced) and from type II (14.5%), whereas none of the type III embryos resulted in a pregnancy. In the authors' experience, using the DMSO protocol, the best pregnancy rates were achieved when replacing 8-cell embryos of high morphologic prefreezing quality. No statistically significant difference could be demonstrated, however, in implantation rates between 8-cell and 4-cell embryos, or between synchronously and asynchronously dividing concepti.