The pustular dermatitis associated with small bowel bypass surgery and the cutaneous manifestations of inflammatory bowel disease are well known and generally assumed to be due to the absorption of microbial antigens from the bowel. Monomeric serum IgA is assumed to originate in the gastrointestinal tract, and circulating IgA immune complexes, as seen in dermatitis herpetiformis, should make us suspicious of a gastrointestinal tract source. These circulating immune complexes and perhaps polyclonal increases in serum IgA may be the result of minor perturbations of mucosal permeability or the failure of locally produced dimeric serum IgA to inactivate bacterial or dietary antigens. Such disparate entities as Reiter's syndrome, psoriasis, pyoderma gangrenosum, and ankylosing spondylitis, as well as the pustular eruptions of Behçet's syndrome, pustular psoriasis, and lithium therapy, may share this common pathogenesis.