While the response to stress has been linked to alterations in immune function, few studies have examined the relationship of stress to disease in children. In this study of 169 children 5 to 10 years of age, parent stress at baseline was linked to illness in their children over the subsequent year. For every 1 unit increase on the stress scale, there was an 11% increase in the frequency of illness and a 36% increase in the rate of febrile illness. This was especially true for psychiatric stress in the parents. Long-term family stress was associated with increased natural killer cell function in children. The specific mechanisms linking family stress to child illness are not known.