Although IgA nephropathy (IgAN) is recognized worldwide as the most common primary glomerulonephritis, the prevalence of this disease among American blacks is strikingly low despite the frequency of other renal disorders. We have previously described the clinical features of 27 black patients enrolled in a multicentre IgAN database; in this paper we report several immunological parameters of the disease in this population. Quantification of serum immunoglobulins revealed significantly higher concentrations of total IgA, IgAl and IgA2 (P=0.0001, 0.002 and 0.005 respectively) in the patients, but no significant increases in IgG or IgM. Examination of immunoglobulin synthesis by peripheral blood lymphocytes indicated relatively few differences in the secretion of immunoglobulins by patients compared to healthy American blacks. The spontaneous production of total IgA, IgA1, and IgA2 in patients was depressed compared to the control subjects (P=0.02, 0.04, 0.03,), yet the ratio of IgA1:IgA2 was normal. Stimulation with poke-weed mitogen enhanced secretion of immunoglobulin in both subject groups. However, a significantly greater IgA1:IgA2 ratio was noted in the patients (P=0.002). Circulating immune complexes containing C3 and IgA as well as C3 and IgM were elevated in the patients (P=0.0006, 0.0003 and 0.02, respectively). These immunological aberrancies did not correlate with clinical manifestations of disease. These data suggest the immune abnormalities of black IgAN patients are similar to, but not identical with, those of white patients. © 1994 European Dialysis and Transplant Association-European Renal Association.