Patients with SBS demonstrate elevated lipopolysaccharide (LPS)-specific IgA levels in serum. LPS-specific IgM, IgA, and IgG were quantitated in serum obtained from clinically stable adults with PN-dependent severe SBS (n = 23) and compared with samples obtained from 48 healthy adults, as outlined in methods. Serum titers of LPS-specific IgA were modestly, but significantly, increased in SBS patients compared with control subjects (P < 0.006). In contrast, anti-LPS IgG and IgM levels in serum were similar in SBS patients and control subjects.