Giardia lamblia infestation can cause severe diarrhea and malabsorption, and the diagnosis is usually made by identification of cysts in the feces, but small intestinal biopsy or smears may be required. A wide spectrum of roentgen changes may be seen. In patients with a normal immune status, the small bowel is normal or shows an inflammatory bowel disease pattern. Eradication of the parasite reverses these changes. In some patients with IgA deficiency, nodular lymphoid hyperplasia occurs, and this is usually not reversible. Other patients with hypogammaglobulinemia or dysgammaglobulinemia and giardiasis may show a sprue pattern. This pattern most often persists after eradication of the parasite. Although the triad of giardiasis, IgA deficiency, and nodular lymphoid hyperplasia has a particularly high association, these, together with diarrhea, malabsorption, and various altered immune states may occur in any combination.