Couples at high risk of passing on a serious genetic condition to their offspring have the opportunity to use Preimplantation Genetic Diagnosis (PGD) to diagnose a specific genetic disease on embryos obtained through in-vitro fertilisation (IVF) before a clinical pregnancy has been established. This article reports the experience of our Centre, from 1999 to March 2004, in PGD for single gene disorders (SGD), describing strategies and overall outcome data of 219 PGD cycles performed on fresh cleavage embryos of 156 couples for 22 different genetic conditions. The single gene defect investigated were autosomal dominant (n=13), autosomal recessive (n=127), or X-linked disorders (n=17). Fifty-five cycles, for 42 couples, were also performed for SGD combined with HLA matching. A total of 1648 embryos were biopsied, in 1265 (76.8%) of which two blastomeres were removed for analysis. PCR amplification was performed on 2913 blastomeres, obtaining a successful amplification in 2678 (91.9%) cells. Diagnosis was achieved for 1536 (93.2%) embryos, 320 of which where transferred to the patients in 156 of the 219 cycles performed. Forty-seven pregnancies were established (30.0% per transfer). All ongoing pregnancies were confirmed to be unaffected by conventional prenatal diagnosis and went to term without complications, resulting in the birth of 31 healthy babies. The present results, complementing other similar experiences on PGD, confirm once again the feasibility of the procedure, providing the opportunity for couples who have a known genetically transmittable disease to start a pregnancy with the knowledge that their child will be unaffected with a genetic condition.