In the United States, increasing numbers of persons are donating kidneys to their spouses. Despite greater histoincompatibility, the survival rates of these kidneys are higher than those of cadaveric kidneys. We examined the factors influencing the high survival rates of spousal-donor kidneys. Kidney-transplant data from the United Network for Organ Sharing Renal Transplant Registry were used to calculate graft-survival rates with Kaplan-Meier analysis. The three-year survival rates were 85 percent for kidneys from 368 spouses, 81 percent for kidneys from 129 living unrelated donors who were not married to the recipients, 82 percent for kidneys from 3368 parents, and 70 percent for 43,341 cadaveric kidneys. The three-year survival rate for wife-to-husband grafts was 87 percent, which was the same as for husband-to-wife grafts if the wife had never been pregnant. If the wife had previously been pregnant, the three-year graft-survival rate was 76 percent (P = 0.40). The three-year graft-survival rate among recipients of spousal grafts who did not receive transfusions preoperatively was 81 percent, as compared with 90 percent for recipients who received 1 to 10 transfusions preoperatively (P = 0.008). The superior survival rate of grafts from unrelated donors could not be attributed to better HLA matching, white race, younger donor age, or shorter cold-ischemia times, but might be explained by damage due to shock before removal in 10 percent of the cadaveric kidneys. Spouses are an important source of living-donor kidney grafts because, despite poor HLA matching, the graft-survival rate is similar to that of parental-donor kidneys. This high rate of survival is attributed to the fact that the kidneys were uniformly healthy.