As rationing of health care services becomes an increasing reality, the pressure to justify interventions such as nutrition support will intensify. The establishment of clinical practice guidelines is one means of providing practitioners with such justification, but clinical practice guidelines for nutrition support cannot be based primarily on prospective randomized trials. This situation arises as the result of limitations specific to nutrition support whereby the most malnourished patients-those who appear most likely to show a benefit from the treatment-cannot be randomized to a no feeding group and are therefore excluded from participation in the study. As the result of this limitation, marginal candidates for nutrition support have been included in some trials, potentially masking the benefits of this treatment. An additional problem limiting present interpretation of published reports of randomized trials in nutrition support is the fact that ongoing research continues to yield improvements in the clinical practice of nutrition support. Thus the nutrition support group in such trials may not have received this treatment according to current practice. The A.S.P.E.N. Guidelines, based on both randomized prospective trials and other types of evidence, represent an important contribution to the practice of nutrition support. Testing of the performance of these and other guidelines in clinical practice and further outcomes research will be important steps toward revision and improvement of nutrition support, but may be difficult to achieve in the near future.