Protein-energy malnutrition (PEM) has been known to cause several organ damages or dysfunctions. In this study, using optic method, platelet aggregation (PA) analyses were performed in patients with PEM. Adenosine diphosphate (ADP)- and collagen-induced PA were significantly decreased in the patients with PEM. The low values reached normal when the patients caught up the optimal growing curve for their age. PA with ristocetin and epinephrine were not significantly altered in the patients. Conversely, mean platelet count was increased at the beginning compared to those of after therapy and to the control. These results indicate that some platelet functions (ADP- and collagen-induced platelet aggregations), as well as other body functions, have been decreased in PEM.