ObjectiveThis study examined the effects of pain and stress associated with a dental procedure, root canal treatment (RCT), on natural killer cell cytotoxicity (NKCC) and the subsequent development of symptoms of upper respiratory illness during the following month.MethodsPatients (N= 33) were recruited from those scheduled for RCT appointments. Subjects for a non-RCT comparison group (N= 14) were also recruited from dental clinic patients. Peripheral blood was drawn by use of an indwelling catheter three times: just before RCT, 30 minutes after injection of a local anesthetic, and 30 minutes after RCT (a parallel time course was followed for the comparison group.) Blood was assayed for cortisol and NKCC. Subjects completed a health diary in the month after RCT.ResultsPatients showed a significant increase in NKCC between baseline and RCT and a significant decrease from RCT to after RCT, whereas the comparison group did not. The NKCC following the RCT was negatively correlated with the pain level during RCT (r= −0.48,p< .01) and pain levels 2 and 6 hours after RCT (r= −0.43,p< .05;r= −0.44p< .05, respectively). The patient group reported significantly more illness episodes 2 weeks after RCT than the comparison group (Wilcoxon rank sum = 4.78,p= .03). Discriminant function analysis correctly classified 88% of the subjects into the illness category using predictor variables of post-RCT NKCC, stress, and pain levels during RCT (F(3,21) = 8.23,p< .001).ConclusionsTransitory changes in NKCC associated with pain and stress may be implicated in the development of infectious disease episodes after an acute stressful event.