Common variable immunodeficiency is characterized with B-cell and T-cell dysfunction and hypogammaglobulinemia. Recurrent bacterial infections, such as otitis media, chronic sinusitis and recurrent pneumonia due to diminished immunoglobulin (Ig) levels and impaired antibody production are frequently observed in common variable immunodeficiency. Almost half of the patients with common variable immunodeficiency have problems related to the gastrointestinal system. A 39-year-old woman was referred to our department with the complaint of chronic diarrhea. She had experienced diarrhea without mucus or blood in the last year and had lost 30 kg. In her medical history, she had suffered from recurrent upper and lower respiratory infections like sinusitis, otitis media and pneumonia since childhood. Serum immunoglobulin levels were low. There were no parasites or ova in her stool examinations. Esophagogastroduodenoscopy detected widespread macroscopic nodular appearance on duodenum, and biopsies from the duodenum revealed giardiasis invading the tissue. She was diagnosed as common variable immunodeficiency. After metronidazole therapy and intravenous immunoglobulin infusion was started, her diarrhea attacks ceased and she regained her normal weight. Common gastrointestinal system problems in patients with common variable immunodeficiency are lactose intolerance, lymphoid hyperplasia/diffuse lymphoid infiltration, loss of villi and infection, especially with Giardia lamblia. Giardiasis may lead to severe mucosal flattening and sometimes to lymphoid hyperplasia at the lamina propria of the duodenum. Medical history should be evaluated carefully regarding recurrent respiratory infections. In such cases with chronic diarrhea, common variable immunodeficiency should be kept in mind as a possible cause.