Previous studies in adult populations have demonstrated alterations in immune function after psychologically stressful events, and pediatric research has shown significant associations between stress and various childhood morbidities. However, no previous work has examined stress-related immune changes in children and subsequent illness experience. Twenty children were enrolled in a study on immunologic changes after kindergarten entry and their prospective relationship to respiratory illness (RI) experience. Midway through a 12-week RI data collection period, the October 17, 1989 Loma Prieta earthquake occurred. The timing of this event created a natural experiment enabling us to study possible associations between immunologic changes at kindergarten entry, the intensity of earthquake-related stress for children and parents, and changes in RI incidence over the 6 weeks after the earthquake. Immunologic changes were measured using helper (CD4+)-suppressor (CD8+) cell ratios, lymphocyte responses to pokeweed mitogen, and type-specific antibody responses to Pneumovax, in blood sampled 1 week before and 1 week after school entry. RI incidence was assessed using home health diaries and telephone interviews completed every 2 weeks. RIs per child varied from none to six. Six children showed an increase in RI incidence after the earthquake; five experienced a decline. Changes in helper-suppressor cell ratios and pokeweed mitogen response predicted changes in RI incidence in the postearthquake period (r = .43, .46; p < .05). Children showing upregulation of immune parameters at school entry sustained a significant increase in RI incidence after the earthquake.(ABSTRACT TRUNCATED AT 250 WORDS)