Several studies have shown the existence of an association between celiac disease (CD) and non-Hodgkin's lymphoma (NHL). Our aim was to evaluate the usefulness of the serum anti-tissue transglutaminase (anti-tTG) antibody assay in screening for CD in consecutive NHL patients. In all, 80 consecutive patients (median age 61 years) with a new diagnosis of NHL were included. To compare the frequency of CD and of positive results for the anti-tTG assay, we enrolled 500 blood donors. In all patients serum anti-tTG was determined with two different ELISA: one based on tTG from guinea pig (gp-tTG) and the other based on human recombinant t-TG (h-tTG) as the antigens. Serum anti-endomysial antibodies (EmA) were also assayed. Subjects with positive serum EmA and/or anti-tTG underwent intestinal biopsy for histology study, HLA-DQ phenotype determination, and serum anti-gliadin (AGA) assay. Eight of 80 (10%) NHL patients were positive for anti-tTG ELISA--two of these exclusively for anti-gp-tTG and six for anti-h-tTG (7.5%). None of the 80 NHL patients were positive for serum EmA. The frequency of anti-tTG positivity in the blood donor controls was 2/500 (0.4%), significantly lower than that observed in the NHL patients (P < 0.0001). Both these blood donors were found to have CD. Only in one anti-h-tTG-positive NHL patient was there intestinal mucosa atrophy, and follow-up confirmed a CD diagnosis (CD frequency in NHL patients is 1.2%; versus blood donors: P = 0.4). In all the other seven anti-tTG-positive NHL patients a normal intestinal architecture was found, although, inflammatory infiltration of the lamina propria was observed in four patients. No anti-tTG-positive NHL patients, including the subject diagnosed as having CD, had a family history of CD, and all had normal weight and no signs of malabsorption. Anti-tTG false positive results were associated with a higher frequency of serum autoantibody positivity and T-cell type NHL. In conclusion, NHL patients the anti-tTG assay often gives discordant data with the EmA assay, with a high frequency of anti-tTG false positive results for CD diagnosis.