It has been previously shown that nutritional produces an increment in the activity ofadenosine deaminase (ADA) in thymus, serum and otherfluids in immunonocompromised patients. This study analyzed if ADA activity could be used as a biochemical indicator of nutritional status in populations at nutritional risk. Twenty six women with anorexia nervosa (AN) 14-32 years oíd, and 33 obese children (O) 5-13 years oíd were studied. ADA activity was determinedas described by Giusty and Galante, comparing results with control subjects ofthe same age. The results (x±SD, expressed as IU/L) wereAN: 25.7+8.2 and 21.0 ±4.6*; and 0:27.1+9.1° and 23.0±5.6 in experimental and control individuáis, respectively (*p<0.02). These findings support the hypothesis that ADA activity in serum can be used as afunctional indicator related to the defence mechanism in nutritional studies.