Diamine oxidase (DAO) is an enzyme located almost exclusively in villus tip enterocytes. Its plasma activity is enhanced by intravenous heparin which releases the enzymes from small bowel enterocytes into the blood. Plasma postheparin DAO (PHD) values have been shown to be significantly lower in patients with malabsorption and villous atrophy, thus suggesting that PHD reflects the mature enterocytic mass. In this study we have assayed PHD in five patients with small bowel lymphoma (two with immunoproliferative small intestinal disease [IPSID] and three with non-IPSID lymphoma) associated with malabsorption syndrome and small bowel mucosa atrophy. The PHD test was performed at diagnosis, after partial or complete remission induced by chemotherapy, and during the follow-up. The PHD values, very low at diagnosis (0.66 +/- 0.12 U/ml), increased during chemotherapy and reached the normal range (greater than 3.7 U/ml) when complete remission occurred. The PHD values rapidly and consistently decreased whenever the disease relapsed. Our data indicate that in patients with small bowel lymphoma PHD test is a sensitive marker of small bowel mucosa damage and suggest that it could be useful in monitoring the recovery of mucosal lesions induced by chemotherapy.