A study was made of the HLA in 41 children between the ages of 4 and 13 years, with the parasite Giardia lamblia, of ages between 4 and 13 years. The different histocompatibility antigens were compared in relationship to the clinical and analytical aspects of the patients. Individuals with HLA A1 and/or blood group A have a greater probability of contracting this parasite. Other relationships found were HLA A19 and vomiting. HLA A9 and abdominal pain and HLA B7 with alterations in the gastrointestinal mucous.