BACKGROUND In chronic atrophic gastritis atrophy of the stomach glands leads to intrinsic factor deficit, with consequent failure to absorb vitamin B12 and gastric achylia, which predisposes to Giardia infection which in itself leads to depletion of vitamin B12. We describe the case of a patient with peripheral and central nervous system pathology due to lack of vitamin B12 secondary to the combined effect of these two disorders. METHODS A 54 year old woman consulted us for paraesthesia and weakness of the legs which had been progressive for the previous two years. She presented with tactile hypoaesthesia, hypoparaesthesia, distal hyperreflexia and dysymmetry of the legs, ataxic-spastic gait and a positive Romberg sign. The investigations carried out showed the serum vitamin B12 level to be 3 pg/ml (N: 180-900), hemoglobin 13 g/dl and MCV 111 fl with MCHC 348/dl; neurophysiological studies: compatible with demyelinating motor polyneuropathy. Schilling test: deficit of absorption of vitamin B12 which was corrected on administration of intrinsic factor; gastroscopy; atrophic gastritis which confirmed the morbid anatomy findings. There was also flora containing Helicobacter and massive Giardia infection. Replacement and antibiotic therapy was followed by complete remission of the clinical picture. CONCLUSIONS We emphasize the excellent clinical response to treatment in spite of the time elapsed since onset of symptoms.