Purpose To compare the effects of 2 different media on embryo morphology and development at days 2/3. Method Six hundred seventy-six attempts from 512 couples were included in this prospective auto-controlled study. Sibling oocytes of all couples undergoing an IVF (n=286) or ICSI (n=390) attempt were randomly assigned to either GIII series (Vitrolife) or ISM (Medicult) media. Primary end points were fertilization and embryo morphology rates. Conclusions Embryo morphology at days 2/3 was significantly enhanced when the embryos were cultured in GIII series.