The transfer of blastocysts obtained by co-culture with 'Vero' (African green monkey kidney) cells was offered to infertile couples with the following indications: (i) repeated failure of implantation, (ii) patients in whom multiple pregnancies had to be avoided (malformed uterus or risk of descending uterus), (iii) patients where embryo development potential had to be assessed, and (iv) replacement of supernumerary embryos frozen at the blastocyst stage. In the 142 cycles analysed, the pregnancy rates per transfer were 37.2, 36.3, 13.0 and 13.6% respectively for the couples with indications i-iv. The respective implantation rates per blastocyst were 20.0, 16.7, 7.1 and 9.3%. In patients in whom multiple pregnancies had to be avoided, the transfer of a maximum of two blastocysts gave a pregnancy rate per cycle of 23.5%, without any multiple pregnancies. The freezing of supernumerary embryos at the blastocyst stage allowed us to replace them using simple protocols and to avoid cancellation of the transfer cycles. Embryo co-culture has been found to be an interesting technique for selected indications, making available a good number of blastocysts for transfer. The transfer of blastocysts allowed us to reduce the number of embryos transferred per patient and therefore also reduce the rate of multiple pregnancies (there were no triplet pregnancies in this study). These results need to be confirmed by larger, randomized studies with comparisons to control groups to evaluate the effectiveness of blastocyst transfers.