Background. The aim of our study was to analyse the results of ICSI procedure with respect to the number of embryos transferred 3 or 5 days after fertilization.Methods. The outcome of 1498 day 3 embryo transfers and 364 day 5 embryo (blastocyst) transfers was analysed. The percentage of implanted embryos was compared with regard to the number and quality of embryos transferred. Differences in pregnancy rate between the groups were established with the chi-square test.Results. Implantation occurred in 21.3% (320/1498) of embryos transferred to the uterus on day 3 post fertilization. In the transfer of one, two, three or four embryos, the pregnancy rate increased from 20.0% to 22.1%, 54.5% and 59.0% respectively. In the group with one or two embryos for transfer, the pregnancy rate did not change statistically significantly in correlation with embryo quality. The twin pregnancy rate after the transfer of two good quality embryos was 66.6% (8/12). The presence of at least one good quality embryo among three or four embryos transferred had a statistically significant effect on the singleton pregnancy rate (37.8% and 41.3%). The rate of multiple pregnancies remained low (18.1% and 16.6%). Increasing the percentage of good quality embryos for transfer to three or four had a greater effect on the percentage of multiple implantations (43.1% and 54.5%) than on the pregnancy rate (55.5% and 75.8%). After the transfer of 5-day-old embryos (blastocysts), implantation occurred in 37.3% (136/364) of embryos. After the transfer of one blastocyst, the pregnancy rate was 36.2% (21/58), after the transfer of two blastocysts it was 55.7% (98/176). The twin pregnancy rate after the transfer of two blastocysts amounted to 38.7%.Conclusions. The transfer of blastocyst stage embryos results in a statistically significant increase in the success of the ICSI procedure. In the transfer of two blastocysts, the rate of twin pregnancies remains equally high as in the transfer of three good quality embryos on the third post fertilization day.