The debate about the pathogenicity of Giardia lamblia in man has ended, and th issues regarding the prevalence of clinical and subclinical infections and their nutritonal impact have become the foremost considerations. Giardiasis can produce steatorrhea, maldigestion, and malabsorption of carbohydrates and of vitamins A and B12. The mechanisms of the absorptive dysfunction are not clear, but morphological abnormalities of the intestinal mucosa and/or bacterial overgrowth might play a role. Severe clinical giardiasis can cause "failure to thrive" in young children, but the impact, if any, of subclinical giardiasis on growth in general populations is not well defined. Protein-energy malnutrition appears to predispose to giardial infection, perhaps because of the accompanying hypochlorhydria, immunosuppression, and altered gastrointestinal flora. The lack of a sensitive and noninvasive diagnostic test for human giardial infection limits the investigation of the nutritional correlates of giardiasis.