In a consecutive study of 216 outpatients with chronic inflammatory bowel diseases (CIBD) low serum and erythrocyte folate levels were found in 59% and 26%, respectively. In patients with low folate levels in both serum and erythrocytes, megaloblastic changes in the bone marrow ere found in 67% (29 out of 44). Their folate intakes were borderline. Absorption studies with tritiated folate showed low absorption values on repeated examinations in 23% of the patients with low folate values (9 out of 40), with no relationship to the intake of salazosulphapyridine. In patients with low folate values the reticulocyte count was elevated (related to the dose of salazosulphapyridine), and the 51Cr erythrocyte survival was decreased. It is suggested that folate deficiency in CIBD is of multiple origin: inadequate diet, malabsorption, and chronic drug-induced low-grade haemolysis. The clinical consequence of the findings remains to be evaluated.