The recent near-epidemic incidence of Giardia lamblia infection in visitors to the Soviet Union illustrates the importance of this intestinal flagellate as a cause of diarrhea in travelers worldwide. Clinical states range from the asymptomatic cyst-passing stage, to the chronic or subacute stage mimicking gallbladder or ulcer disease, to the transient or, rarer, persistent acute stage with steatorrhea and substantial weight loss. Symptoms may be related to IgA deficiency. Secondary lactose intolerance may follow eradication of the parasite. Diagnosis is usually based on repeated stool examinations or examination of duodenal contents. Quinacrine hydrochloride is the most effective treatment, but metronidazole and furazolidone are also useful. Contaminated water is the most likely source of infection.