To determine the relationship between malnutrition and immune status in older and younger outpatients, we assessed 179 outpatient men according to nine markers of malnutrition and assigned them to a well nourished or a malnourished group. Immune status was measured by mixed lymphocyte culture, lymphocyte response to three mitogens, neutrophil chemotaxis, and immunoglobulin levels. Each group was further divided into those 65 years and older (28%) and those younger than 65 years (72%). Malnutrition was present in 29% of the younger group and 38% of the older. Malnourished patients had less lymphocyte response to allogeneic cells and to phytohemagglutinin, but more response to pokeweed and higher IgA levels. In the younger group chemotaxis results showed that malnourished persons had higher responses than well nourished; the trend was opposite in the older group. More differences in immune status were found between malnourished and well nourished than between old and young, but more unfavorable responses were found in the older malnourished than in any other group. The study suggests that malnutrition is common among outpatients and that depressed immune responses associated with malnutrition can lead to disease, particularly in the elderly.