This study quantified the concentrations of interleukin-1 alpha (IL-1 alpha) and interleukin-1 beta (IL-1 beta) in the periapical exudates obtained from 69 single-rooted teeth using enzyme-linked immunosorbent assays, and examined their correlation with clinical findings of the involved teeth. Changes in the levels of these factors during root canal treatment were also investigated. The average levels of IL-1 beta (6.57 ng/ml) in periapical exudates were twice that of IL-1 alpha (3.25 ng/ml). The exudates containing pus showed significantly higher IL-1 alpha levels than those from the canals without pus (p < 0.01). The exudates from the canals with small radiolucent areas contained significantly higher IL-1 alpha levels than those from the canals with large radiolucent areas (p < 0.05). The tendency for there to be an increase in the levels of IL-1 alpha and a decrease in the levels of IL-1 beta was observed following root canal treatment. These observations suggest that IL-1 alpha and IL-1 beta are involved in the immunopathogenesis of periapical lesions and that IL-1 alpha and IL-1 beta may play different roles in the healing process of periapical lesions during root canal treatment.