Background: Volumetric alterations in gingival crevicular fluid (GCF) are widely accepted to be associated with periodontal health/disease. The volume/flow of GCF was shown to be affected by an array of methodological factors. However, relatively limited information is available on whether GCF is subject to circadian rhythm. The main aim of the present study is to assess the possible presence/absence of GCF circadian rhythm. The impact of the sampling technique on daytime volumetric variations is also analyzed.