Membranoproliferative glomerulonephritis (MPGN) type III is a chronic progressive renal disease of unknown cause. The diagnosis is based on renal pathologic features (specifically immunofluorescence staining patterns and ultrastructural appearance). Mesangial cell proliferation and subendothelial and subepithelial deposits characterize the renal disease. Although the actual prevalence of this disease is not known, the disease is rare and usually sporadic. The clinical features of MPGN include the nephrotic syndrome and hematuria, with renal dysfunction occurring in approximately 50% of patients. Progression to end-stage renal disease is variable, and some patients exhibit stabilization or even improvement. Here is presented an Irish family in which there are eight affected members in four generations, suggesting autosomal dominant inheritance. This is the only reported family with an inherited form of MPGN type III. To evaluate the disease in this family, a genome-wide scan was performed with a panel of 402 polymorphic microsatellite markers, defining a grid with an average resolution of 10 cM (centimorgans). Significant evidence for linkage was observed on chromosome 1q31-32, with a maximal logarithm of the odds score of 3.86 at theta = 0.00 for microsatellite marker GATA135F02. Recombination events among affected individuals, as detected by haplotype analysis, established a 22-cM minimal candidate region flanked by markers D1S3470 and GATA124F08. The data provide evidence for a gene for familial MPGN on chromosome 1q.