The diagnosis of inherited disease in affected embryos before implantation is now possible using in vitro fertilization (IVF), embryo biopsy and DNA analysis. This enables pregnancies to be established with unaffected embryos following transfer to the uterus avoiding the possibility of terminating a pregnancy diagnosed as affected later in gestation. Pregnancies have been established with female embryos in women carrying X-linked recessive genetic defects affecting boys by identifying the sex of embryos either by DNA amplification of Y-specific sequences or by dual fluorescent in situ hybridization (FISH) with X- and Y-specific probes to interphase nuclei. The predominant ΔF508 deletion causing cystic fibrosis (CF) has also been detected by DNA amplification from single cells and the first pregnancy and birth of a child successfully screened for a single gene defect achieved. The prospects for applying preimplantation diagnosis to other chromosomal and single gene defects are reviewed.