We studied natural killer cell (NK) activity in 20 severely handicapped children and investigated the relationship between NK activity and malnutrition. We divided our patients into two groups with regard to frequency of infections: high frequency group (group 1) and low frequency group (group 2). NK activity in group 1 was low in comparison with group 2. Calorie intake and Rohrer's index were lower in group 1. Serum albumin (Alb) and iron (Fe) in group 1 were also lower than those in group 2, and serum zinc (Zn) and hemoglobin (Hb) were slightly lower in group 1. NK activity was positively correlated with calorie intake, Alb, Zn and Hb. Six patients with low NK activity were given supplementary diet; two patients were given protein-enriched diet; three patients were given protein-enriched diet and zinc sulfate; one patient was given increased diet. NK activity rose in 4 patients. These results indicated that the depression of NK activity was associated with frequent infections and correlated with nutritional conditions of the severely handicapped children.