Henoch-Schönlein purpura (HSP) is a common vasculitis being characterized by the classic tetrad of nonthrombocytopenic palpable purpura, arthritis or arthralgias, gastrointestinal and renal involvement. Antigen-antibody (IgA) complexes activate the alternative complement pathway, resulting in inflammation and small vessels vasculitis. We present the case of a 53 years old Italian woman with HSP who was previously hospitalized for purpura skin lesions of the lower legs and diarrhea; a skin biopsy showed a leukocytoclastic vasculitis with perivascular accumulation of neutrophils and mononuclear cells. She was treated with immunosuppressive therapy. After 8 months she was hospitalized again for a recurrent episode of purpura skin lesions of the lower legs. At age 49 she was affected by obesity (BMI = 41.6 Kg/m2), treated via a bilio-pancreatic diversion that led, within a year, to a BMI reduction (25 Kg/m2). We suppose that bariatric surgery played a role on the development of autoimmune phenomena and that the formation of immunecomplexes is secondary to the excess of intestinal bacterial antigens. A cyclic therapy with Paromomicine 500 mg twice daily and Metronidazole 250 mg twice daily was performed with a clear up of the clinical picture. In medical literature are described numerous complications which include arthritis, erythema nodosum-like lesions, eruptions and other skin manifestations in patients who have undergone jejunocolic bypass. This case report describes for the first time the presence of HSP in a patient with bowel bypass syndrome and it is also able to demonstrate the relationship between the intestinal bacterial overgrowth and the systemic autoimmune system.