Embryonic stem cell research involves the extraction of stem cells from embryos which results in the death of that embryo. The Church opposes this as there is no reason or matter so noble that human beings should be created to be used and destroyed. The Church opposes all direct destruction of human life for any purpose, including research.
The Church does not oppose the use of adult stem cells which can be obtained from adult tissue, umbilical cord blood, and other sources. The use of these cells can be very effective in the treatment of a variety of diseases.