Phylloquinone is a lipid soluble vitamin which is an absolute growth requirement for black-pigmented anaerobes, many of which are implicated in the aetiology of periodontal diseases. This cross-sectional study aimed to detect the levels of phylloquinone in GCF from healthy and diseased sites in subjects with adult periodontitis, in order to investigate further its potential role in the disease process. The sample consisted of eighteen patients with adult periodontitis. Periodontal probing depths, attachment levels and gingival indices were recorded from one healthy and one diseased site in each subject. GCF was sampled and the amount of phylloquinone in each sample was determined using reverse-phase high performance liquid chromatography coupled to electrochemical detection. The mean amount of phylloquinone in accumulated GCF from diseased sites was 406 pg/site and 80 pg/site from healthy sites (p=0.013). When the amounts of phylloquinone in GCF were expressed as concentrations the values were 228 ng/ml and 3350 ng/ml for diseased and healthy sites respectively (p=0.084). These findings suggest the levels of phylloquinone in GCF differs in periodontal health and disease in subjects with adult periodontitis. The total phylloquinone at diseased sites may provide the nutritional requirements favouring the growth of black-pigmented anaerobes.