Anti-glomerular basement membrane (GBM) disease is caused by an autoantibody directed against an epitope on the alpha 3 chain of type IV collagen. Animal models demonstrate that the higher the affinity of such antibodies, the greater the degree of glomerular injury. Affinity maturation (the process whereby somatic mutation followed by antigen selection leads to an increase in affinity of antibody) might therefore be of pathogenic significance if it occurs in human anti-GBM disease. We have examined serial samples from nine patients with anti-GBM disease and looked for evidence of changing functional affinity by measuring the inhibition of binding produced by the mild chaotrope diethylamine (DEA) in an anti-GBM antibody ELISA. Seven patients showed no change in the inhibition produced by DEA with time. Two patients showed an apparent decrease with time in the inhibition produced by DEA; this apparent increase in functional affinity proved, on further investigation, to represent simply the loss of anti-GBM antibodies. These results may imply that affinity maturation has been completed by the time that patients present with anti-GBM disease. If there had been evidence for a further increase in functional affinity after this point then this might have added extra urgency to the need for removal of these autoantibodies as part of treatment.