Natural killer cells capable of lysing herpes simplex virus type 1 (HSV-1)-infected fibroblasts were studied in three groups of patients unusually susceptible to severe herpes-virus infections. Cord blood was evaluated because of the known susceptibility of neonates to disseminated infections due to herpes simplex virus type 2 at birth. Only 30% of the cord blood specimens tested demonstrated normal lysis of HSV-1-infected fibroblasts and a normal increment in the lysis of infected over uninfected cells. Five out of six patients with Wiskott-Aldrich Syndrome (WAS) also were found to have abnormally low responses by these criteria. The one WAS patient with normal responses had had little difficulty with infections and had survived much longer than usual. Five patients with severe herpesvirus infections and no known primary cellular immunodeficiency had natural killer cell function significantly below normal (P less than 0.001). These data suggest that natural killer cells probably play an important role in human resistance to herpesvirus infection and that deficiencies of this system may result in unusual susceptibility to herpesvirus infections.