Renal dysplasia and agenesis may be a familial disorder. We report the familial occurrence of unilateral renal agenesis and proteinuria that, at least in one case, was related to focal glomerulosclerosis. Whether these abnormalities are related to an intrinsic abnormality in the remaining kidney, hyperfiltration injury, systemic hypertension, or some other poorly defined factor is unclear at present. However, this report, along with previous case reports of familial renal agenesis, suggests that ultrasonographic screening of first-degree relatives of patients with renal agenesis is appropriate. Whether treatments such as dietary protein restriction, use of angiotensin-converting enzyme inhibitors, or other therapeutic interventions will have a beneficial effect in asymptomatic individuals with unilateral renal agenesis remains to be determined.