Exactly 70 years ago mercaptopurine (1951) was discovered by Gertrude Elion as a novel treatment option for acute leukemia. A total of three thiopurines (also thioguanine (1950) and azathioprine (1957)) were developed over time. These immunosuppressive drugs were also successfully introduced a few decades later to prevent rejection of transplanted organs and to treat several autoimmune diseases. For the discovery of thiopurines and other antimetabolite drugs she was rewarded in 1988, together with George Hitchings and James Black, with the Nobel Prize in Physiology or Medicine. Important steps have been made in recent years to unravel its metabolism, mode of action and pharmacogenetics. Till today thiopurine (based) therapy remains an essential immunosuppressive approach in treating patients with inflammatory bowel disease.