We used a sensitive probe of pancreatic dysfunction, serum immunoreactive cationic trypsinogen, to study 50 infants and children with varying degrees of malnutrition. Patients were classified into subgroups according to the severity of malnutrition. Mean serum trypsinogen concentration was significantly elevated in 25 patients with "severe" malnutrition (77.4 +/- 42.0 ng/ml, P less than 0.001) and in 23 with "moderate" malnutrition (55.2 +/- 16.1 ng/ml, P less than 0.02) compared with the mean value (32.5 +/- 10.4 ng/ml) for well-nourished controls. The level of circulating trypsinogen tended to rise with increasing severity of malnutrition. There was no relationship between serum trypsinogen and other variables such as age, specific diagnosis, or mode of feeling. Elevated serum trypsinogen levels could not be attributed to renal disease or cystic fibrosis. In patients who showed an improvement in nutritional status, serum trypsinogen tended to revert toward normal. Elevated serum trypsinogen values in acutely malnourished infants and children may result from pancreatic acinar cell damage or regurgitation of enzymes from obstructed pancreatic ducts.