To assess the relationship between pronuclear scoring and day-3 embryo quality and pregnancy outcome and to determine the clinical value of pronuclear stage scoring system in human in vitro fertilization-embryo transfer (WF-ET) program, a pronuclear scoring system was used to score zygotes 16-20 h after insemination during conventional WF or intracytoplasmic sperm injec- tion (ICS1). The embryos were classified into groups Z1, Z2, Z3 and Z4. Comparisons were made of the rates of arrested embryos and excellent embryos on day 3. Comparisons of pregnancy outcome were made only in those patients in whom cohorts of similarly Z-scored embryos were transferred. The results showed that there were less arrested embryos and more excellent embryos on day 3 in groups Z1 and Z2 than those in group Z3 and Z4. More embryos arrested and less excellent embryos developed in group Z4 than group Z3. The clinical pregnancy rates resulting from the transfer of single pronuclear score homologous embryo types were similar among groups Z1, Z2 and Z3. Implanta- tion rates of group Z1 were higher (P<0.05) than that of group Z3. These findings suggests that pro- nuclear scoring can predict developmental ability on day 3 and implantation potential. A evaluation that combines the Z-score and day 3 embryo morphology is useful in the determination of the most viable embryos and the number of embryos for transfer.