Depending upon your age, you might remember Jewish tradition on the topic of organ donation as very different from how it actually is today. Once opposed, Jewish law and practice on organ donation has changed dramatically, which is the beauty of Judaism as a living, evolving tradition. Now that organ transplantation is a highly successful way to save a life, organ donation has been deemed an obligatory act, a mitzvah chiyuvit, by every major branch of Judaism.
Now, it is important to note that some Orthodox leaders differ on how to determine the time of death, and prefer a point later than brain death, which results in some organs being rendered unusable but even in that case, the kidneys, barring kidney disease, remain transplantable after death. Accordingly, not to bequeath at least some of your organs has become a transgression of the mitzvah of pikuakh nefesh, “saving a life.”