The surplus of fresh embryos obtained at our laboratory within the in-vitro fertilization programme led us to implement human embryo freezing--thawing techniques from August, 1985. Thus far, 20 freezing cycles (45 embryos) have been carried out using this method. Two fast-freezing techniques were used and one fast-thawing method in 1 M sucrose. In the first freezing technique, dimethyl sulphoxide (DMSO; 1.5 M) was added in a step-wise fashion and seeding was done directly on the embryo compartment. In the second, DMSO (1.5 M) was added in one step and seeding was carried out away from the embryo compartment. In both techniques, embryos were stored in 0.25-ml plastic straws in phosphate-buffered saline (PBS) containing 15% maternal serum. Embryos were replaced during the natural cycle, 48-50 h following the LH peak. Three pregnancies were obtained from six transfer cycles. Both freezing techniques were successful, but stepwise addition of DMSO and seeding the embryo compartment directly appeared to be more successful. One pregnancy was a pre-clinical abortion, the second a clinical abortion at 8 weeks, and the third is ongoing at 19 weeks. These are the first pregnancies in the American continent achieved with frozen embryos.