Some autoimmune diseases tend to cluster within families, patients with one such disease showing an increased tendency to develop certain other autoimmune diseases within the same group. We have investigated the possibility that this clustering arises because some patients produce autoantibodies which cross-react with antigenic determinants which are shared by diverse tissues. Our data indicate that this is not the case, at least so far as thyroid and gastric autoimmune diseases are concerned. This observation increases the likelihood that the real explanation for such clustering concerns sharing of idiotypic determinants by the different pathogenic clones involved or by their precursors.