Cytokines play an important role in the pathology associated with chronic inflammatory diseases. One of these cytokines, interleukin 6 (IL-6) is a major mediator of the host response to tissue injury, infection and bone resorption. In the present study, gingival crevicular fluid (GCF) level of IL-6 was determined in patients with non-insulin dependent diabetes mellitus (NIDDM) with periodontitis, adult periodontitis, and healthy controls by use of an enzyme linked immunosorbent assay (ELISA). Twenty-four NIDDM patients with periodontitis, twenty-four adult periodontitis and twenty-four healthy controls were selected for the study. GCF sampling was performed on the vestibular aspects of maxillary incisors and canine teeth. Plaque index (PI), gingival index (GI), gingival bleeding time index (GBTI), probing depth (PD) and probing attachment levels (PAL) were recorded from each sampling area and also the entire dentition. NIDDM and adult periodontitis patients had numerous sites with radiographic evidence of alveolar bone resorption, loss of attachment and pocket depth greater than 3 mm. The mean GCF IL-6 level was 2.43 +/- 0.97 ng/ml in NIDDM patients, 1.31 +/- 0.92 ng/ml in adult periodontitis and 0.62 +/- 0.58 ng/ml in healthy subjects, respectively (p < 0.05). GCF IL-6 levels were markedly higher in NIDDM and adult periodontitis groups compared to the healthy controls. No correlation was found between GCF IL-6 levels and all clinical parameters. These findings suggested that GCF IL-6 levels were significantly higher in the area of inflammation and periodontal destruction locally. The high IL-6 levels in NIDDM patients might be due to different microbial flora in periodontal pockets and altered immune system. Future studies are needed to evaluate the complex interaction among IL-6 GCF levels, host response and local microbial environment in the NIDDM patients.