Malnutrition leads to cellular and humoral immunological response disorders. A study protocol on malnourished patients has been designed in order to assess immune response mechanisms that may be altered due to malnutrition and may or may not recover once normal metabolic conditions have been restored prior to starting the patient on parenteral feeding and control. The immune response was assessed especially regarding immunoglobulin, lymphocyte subsets and mitogen response levels. It is important to know the different malnutrition-related immunologic disorders, disorders secondary to diseases that hinder correct oral feeding, and immunologic tests may be used to assess nutritional parameters. The malnourished patients included in this trial has to be free of any other immunologic or neoplastic disease, and not receive immune response suppression therapy. The trial patients showed lower total and relative CD3 and CD4 lymphocyte values at the onset of the study, although the former lymphocyte subset recuperated sooner at the expense of increasing CD8 lymphocytes, while CD4 lymphocytes still remained low after 15 days of parenteral nutrition. Immunoglobulin levels remained within normal limits. The mitogen response capacity, which was sensibly low at the beginning, recuperated in 50% of the cases treated with parenteral nutrition. This parameters may be used as an index to assess the nutritional status of these patients. The cases studies allowed us to conclude that there was a decrease in CD4 cells and mitogen response in malnourished patients. After 15 days of parenteral nutrition, the cells did not recuperate but their function measured in terms of phytohemagglutinin, was normal in 50% of the cases.