AIM: To demonstrate the 18F-fluorodeoxyglucose positron emission tomography (18F-FDG PET) findings in patients with non-Hodgkins lymphoma (NHL) involving the gastrointestinal (GI) tract and the clinical utility of modality despite of the known normal uptake of FDG in the GI tract.METHODS: Thirty-three patients with biopsy-proven gastrointestinal NHL who had undergone FDG-PET scan were included. All the patients were injected with 10-15 mCi FDG and scanned approximately 60 min later with a CTI/Siemens HR (+) PET scanner. PET scans were reviewed and the maximum standard uptake value (SUVmax) of the lesions was measured before and after the treatment,if data were available and compared with histologic diagnoses.RESULTS: Twenty-five patients had a high-grade lymphoma and eight had a low-grade lymphoma. The stomach was the most common site of the involvement (20 patients). In high-grade lymphoma, PET showed focal nodular or diffuse hypermetabolic activity. The average SUVmax±SD was 11.58±5.83. After the therapy,the patients whose biopsies showed no evidence of lymphoma had a lower uptake without focal lesions.The SUVmax±SD decreased from 11.58±5.83 to 2.21±0.78. In patients whose post-treatment biopsies showed lymphoma, the SUVmax±SD was 9.42±6.27. Low-grade follicular lymphomas of the colon and stomach showed diffuse hypermetabolic activity in the bowel wall (SUVmax 8.2 and 10.3, respectively). The SUVmax was 2.02-3.8 (mean 3.02) in the stomach lesions of patients with MALT lymphoma.CONCLUSION: 18F-FDG PET contributes to the diagnosis of high-grade gastrointestinal non-Hodgkins lymphoma,even when there is the normal background FDG activity. Furthermore, the SUV plays a role in evaluating treatment response. Low-grade NHL demonstrates FDG uptake but at a lesser intensity than seen in high-grade NHL.