BACKGROUND This study investigates whether gingival crevicular fluid (GCF), saliva, and plasma levels of interleukin-33 (IL-33) can differentiate individuals with chronic periodontitis from individuals with healthy periodontium. METHODS GCF, whole saliva, and plasma samples together with full-mouth clinical periodontal recordings were obtained from 32 otherwise healthy, non-smoker chronic periodontitis individuals and 25 systemically and periodontally healthy, non-smoker individuals. IL-33 levels in the biofluid samples were determined by enzyme-linked immunosorbent assay. Data were tested statistically by Mann-Whitney U test. RESULTS The GCF concentrations of IL-33 were significantly lower in chronic periodontitis individuals than in healthy individuals (P <0.0001), whereas the total amounts in GCF samples were similar (P >0.05). The salivary and plasma contrations of IL-33 were indifferent in the two study groups (P >0.05). CONCLUSIONS According to the present findings, the GCF, saliva or plasma levels of IL-33 could not differentiate chronic periodontitis individuals and periodontally healthy individuals. Larger-scale intervention studies may better clarify this issue.