Numerous studies document that women constitute the majority of living kidney donors, but the reasons behind the disparity in donation rates between men and women remain obscure. We studied this issue by gathering data on family members of living donor allograft recipients at a single large center over a 5-year period (n = 144). By considering all potential donors (spouses and first-degree relatives) within each recipient's immediate family, we determined that men and women are excluded as donors at approximately similar rates on the basis of medical condition or known blood group type A, type B, type O incompatibility, and that a greater percentage of acceptable female donors (28.3%) compared with men (20.3%) go on to donate a kidney (P: = 0.027). However, when only first-degree relatives are considered, the difference in donation rate between men and women becomes nonsignificant (26.9% of women versus 22.2% of men; P = 0.229). Among spouses, the gender disparity in donation rate is greater (36% of wives versus 6.5% of husbands who are acceptable donors go on to donate a kidney; P = 0.003). Evidence that economic factors may contribute to the overall gender disparity is also presented. In conclusion, the gender disparity among living kidney donors observed in our population can be largely attributed to an overwhelming predominance of wives among spousal donors. Possible explanations and potential interventions to address underrepresentation of male donors are discussed.