Volatile sulfur compounds (VSC) are a family of gases which are primarily responsible for halitosis, a condition in which objectionable odors are present in mouth air. Although most patients perceive this condition as primarily a cosmetic problem, an increasing volume of evidence is demonstrating that extremely low concentrations of many of these compounds are highly toxic to tissues. VSC may, therefore, play a role in the pathogenesis of inflammatory conditions such as periodontitis. Since these compounds result from bacterial putrefaction of protein, investigations have been conducted to determine whether specific bacteria are associated with odor production. Two members of this family, hydrogen sulfide (H2S) and methyl mercaptan (CH3SH), are primarily responsible for mouth odor. Although many bacteria produce H2S, the production of CH3SH, especially at high levels, is primarily restricted to periodontal pathogens. Direct exposure to either of these metabolites adversely affects protein synthesis by human gingival fibroblasts in culture. However, methyl mercaptan has the greatest effect. Other in vitro experiments have demonstrated that cells exposed to methyl mercaptan synthesize less collagen, degrade more collagen, and accumulate collagen precursors which are poorly cross-linked and susceptible to proteolysis. CH3SH also increases permeability of intact mucosa and stimulates production of cytokines which have been associated with periodontal disease. VSC, and in particular methyl mercaptan, are therefore capable of inducing deleterious changes in both the extracellular matrix and the local immune response of periodontal tissues to plaque antigens. This article reviews these data and emphasizes the potential importance of VSC in the transition of periodontal tissues from clinical health to gingivitis and then to periodontitis.