The role of interleuking-8 (IL-8), a neutrophil-attracting and-activating cytokine, was investigated in gingival crevicular fluid (GCF). ELISA was used to detect the levels of IL-8 in GCF collected from 54 adult periodontitis (AP) patients (105 teeth) and 24 healthy subjects (54 teeth). The results showed that 1. The role of IL-8 was dependent upon the concentration of IL-8 in GCF. IL-8, in the low concentration (< or = 30 micrograms/L), was positively correlated with bleeding index (r = 0.36, P < 0.01). While in the high concentration (> 30 micrograms/L), was negatively correlated with bleeding index and probing depth (r = -0.54 and r = -0.65 respectively, P < 0.01). In the majority of periodontitis sites (91%), the concentration of IL-8 in GCF were lower than 30 microliters/L. IL-8 most likely acted as pro-inflammatory factor in these teeth. 2. IL-8 was a two-way regulator of inflammation, pro-inflammation and anti-inflammation. The threshold of IL-8 between inducing and suppressing inflammation was approximately 30 micrograms/L. Within the confined range, IL-8 concentration greater than 30 micrograms/L could be inflammation suppressive, while a less-than-30 micrograms/L concentration of IL-8 might become inflammation inducing.