OBJECTIVE. Intestinal infection with Giardia lamblia may lead to therapy-resistant, long-lasting post-giardiasis irritable bowel syndrome (IBS). We report two open pilot studies aiming to treat this condition, using either antibiotics or bacterio-therapy. MATERIAL AND METHODS. Twenty-eight patients with persistent abdominal symptoms, following clearance of G. lamblia infection, were investigated. Eighteen received treatment with rifaximin plus metronidazole (8-10 days) whereas 10 received a suspension of live faecal flora, installed into the duodenum during gastro-duodenoscopy. Customary abdominal symptoms and symptoms following a lactulose breath test were quantified by questionnaires. Hydrogen and methane production after lactulose were analysed in expired air and excretion of fat and short-chain fatty acids (SCFAs) was examined in faeces. RESULTS. As compared with pre-treatment values, total customary symptom scores were barely significantly reduced (p = 0.07) after antibiotics, but were highly significantly reduced (p = 0.0009) after bacterio-therapy. However, symptom improvement following bacterio-therapy did not persist 1 year later. Hydrogen breath excretion was slightly reduced after antibiotics, but not after bacterio-therapy. Compared with healthy persons, faecal excretion of fat was significantly increased in Giardia-cured patients. SCFAs were increased in the bacterio-therapy group, and were not influenced by therapy. CONCLUSIONS. Both antibiotics and bacterio-therapy were ineffective with respect to cure of post-giardiasis IBS. High faecal excretion of fat and SCFAs suggests that intestinal malabsorption of fat and carbohydrates may play a role in the IBS-like complaints of these patients.