Constipation is common but diarrhea is very rare in myxedematous patients. We report a young woman with myxedema in whom chronic diarrhea was the dominant symptom. A fasting breath hydrogen test was positive, indicating bacterial overgrowth as the cause of the diarrhea. Antibiotic treatment was successful. We conclude that bacterial overgrowth due to hypomotility may be the etiology of the diarrhea in such patients.