Familial cases of mesangial IgA glomerulonephritis (MGN) have raised the possibility of a genetic control in this disease. In 50 patients with MGN, diagnosed on renal biopsy, and in 105 controls, we have compared the distribution of HLA antigens (A and B loci). We found a significant increase in the frequency of HLA Bw35 antigen in the patient group compared with controls (36% versus 13%: p less than 0.02). There was no significant difference between the Bw35 positive and negative MGN subgroups, in clinical, serological, and pathological data. Both subgroups had elevated mean serum IgA levels (154% of normal), and also mean serum IgM levels (146%). However, the follow-up data exhibited a significantly worse prognosis (p less than 0.01) in the Bw35 positive subgroup: 9 out of 18 patients versus 4 out of 32 progressed to chronic renal failure (serum creatinine greater than 1.5 mg/dl). We have established a genetic linkage between the HLA complex and the occurrence of MGN. The Bw35 antigen may serve as a marker (risk of disease = 4), in particular for poor prognosis cases.