Loss of renal mass has been associated with the development of glomerular sclerosis in animals and human beings. The pathophysiology of this renal injury is unknown, but glomerular sclerosis in animals can be aggravated or accelerated following exposure to nephrotoxic antibodies, puromycin aminoglycoside or renal irradiation. We describe here the outcome of the first renal transplant performed in the British Commonwealth. Glomerular sclerosis occurred in identical twins who were kidney donor and recipient, renal failure occurring 14 and 16 years after transplantation, respectively. Examination of these twins and all living immediate family members showed that six of the seven family members (both twins, their mother, and three sisters) had increased concentrations of circulating immune complexes, decreased total hemolytic complement, and low or borderline concentrations of C4. Only twins with single kidneys had detectable renal disease. Other preexisting causes of renal disease in these twins that would account for the glomerular sclerosis could not be identified. We suggest that a familial immune defect contributed to the development of glomerular sclerosis in these twins who were predisposed to renal disease due to loss of renal mass.