IgA nephropathy, a condition thought to cause slowly progressive renal damage, is frequently associated with high serum IgA levels. As phenytoin sodium lowers serum IgA concentrations, a controlled trial of therapy with this drug was conducted over a two-year period in patients with IgA nephropathy. Despite significant depression of serum IgA concentrations in the treatment group, there was no significant change in any other clinical, biochemical or pathological parameter, in either control or treatment groups. Indeed, there was evidence for a slow progression of renal damage in both groups. These observations suggest that the elevated serum IgA concentrations in IgA nephropathy are not of primary pahtogenetic significance but are rather a consequence of a basic abnormality in antigen processing and IgA production.