Periodontitis is a chronic bacterial infection of the supporting structures of the teeth. The host response to infection is an important factor in determining the extent and severity of periodontal disease. Systemic factors modify periodontitis principally through their effects on the normal immune and inflammatory mechanisms. Several conditions may give rise to an increased prevalence, incidence or severity of gingivitis and periodontitis. The effects of a significant number of systemic diseases upon periodontitis are unclear and often it is difficult to causally link such diseases to periodontitis. In many cases the literature is insufficient to make definite statements on links between certain systemic factors and periodontitis and for several conditions only case reports exist whereas in other areas an extensive literature is present. A reduction in number or function of polymorphonuclear leukocytes (PMNs) can result in an increased rate and severity of periodontal destruction. Medications such as phenytoin, nifedipine, and cyclosporin predispose to gingival overgrowth in response to plaque and changes in hormone levels may increase severity of plaque-induced gingival inflammation. Immuno-suppressive drug therapy and any disease resulting in suppression of the normal inflammatory and immune mechanisms (such as HIV infection) may predispose the individual to periodontal destruction. There is convincing evidence that smoking has a detrimental effect on periodontal health. The histiocytoses diseases may present as necrotizing ulcerative periodontitis and numerous genetic polymorphisms relevant to inflammatory and immune processes are being evaluated as modifying factors in periodontal disease. Periodontitis severity and prevalence are increased in diabetics and worse in poorly controlled diabetics. Periodontitis may exacerbate diabetes by decreasing glycaemic control. This indicates a degree of synergism between the two diseases. The relative risk of cardiovascular disease is doubled in subjects with periodontal disease. Periodontal and cardiovascular disease share many common risk and socio-economic factors, particularly smoking, which is a powerful risk factor for both diseases. The actual underlying aetiology of both diseases is complex as are the potential mechanisms whereby the diseases may be causally linked. It is thought that the chronic inflammatory and microbial burden in periodontal disease may predispose to cardiovascular disease in ways proposed for other infections such as with Chlamydia pneumoniae. To move from the current association status of both diseases to causality requires much additional evidence. Determining the role a systemic disease plays in the pathogenesis of periodontal disease is very difficult as several obstacles affect the design of the necessary studies. Control groups need to be carefully matched in respect of age, gender, oral hygiene and socio-economic status. Many studies, particularly before the aetiological importance of dental plaque was recognised, failed to include such controls. Longitudinal studies spanning several years are preferable in individuals both with and without systemic disease, due to the time period in which periodontitis will develop.