The most important transplantation antigen system in solid organ transplantation is the ABO histo-blood group system. Crossing the ABO barrier in solid organ transplantation is usually not done except for emergency liver transplantations. Early experiences of crossing the ABO barrier in renal transplantation were very disappointing. In the 1970s, clinical trials were started transplanting kidneys of subgroup A2 into blood group O recipients. The tissues of the A2 subgroup expresses reduced amount of A antigens compared to subgroup A1 and the recipients had no special pretreatment and standard immunosuppression. A number of early graft losses were experienced but the trial also resulted in several long time surviving grafts. A few centres have adapted the concept of A2 to non A kidney transplantations with successful results, when the recipient anti-A titres are low or reduced prior to transplantation. In the early 1980s one group successfully transplanted A1 and B kidneys from living related donors across the ABO-barrier using an immunosuppressive protocol consisting of quadruple drugs and splenectomy and this protocol was adapted by a few other groups. In Japan, where cadaver donors are available in very limited number, the largest number of ABO-incompatible transplantations have been performed. Altogether more than 300 ABO-incompatible kidney transplantations have been performed in more than 40 centres since 1989. ABO-incompatible liver transplantations have been performed mainly in emergency cases and the results have generally been inferior to ABO-compatible grafts. In children below the age of three years, liver transplantations across the ABO-barrier have been quite successful especially with living related donors. Very few ABO-incompatible heart/heart-lung/lung-transplantations have been reported with a few successful cases, but the majority have been failures. Recently a series of ABO-incompatible heart transplants performed in small children have been reported with a high success rate.