The purpose of this review was to investigate evidence for a possible etiological association between oral health and pneumonia or other respiratory diseases. The following data sources were used: Ovid MEDLINE (In-Process & Other Non-Indexed Citations, Daily Update, and OLDMEDLINE); Cumulative Index to Nursing & Allied Health Literature; Evidence Based Medicine of Cochrane Central Register of Controlled Trials; Cochrane Database of Systematic Reviews; Database of Abstracts of Reviews of Effects; EMBASE; Health and Psychosocial Instruments; HealthSTAR; International Pharmaceutical Abstracts; PubMed; and Google Scholar from the earliest record until July 2005. Studies were selected from randomized controlled clinical trials and longitudinal, cohort, case-control, and epidemiological studies. Searches were limited to English language and human studies. A total of 728 articles were searched for relevancy, determined by article title, abstract, and full copy, resulting in a yield of 19 studies that met our inclusion criteria. These articles were read and scored independently by the reviewers to obtain the evidence for this review: 1) the potential risk factors for pneumonia were identified as the presence of cariogenic and periodontal pathogens, dental decay, and poor oral hygiene in five studies; 2) a weak association between periodontal disease and chronic obstructive pulmonary disease (COPD) was identified in four poor to fair studies; and 3) 10 studies were retained providing evidence that interventions aiming to improve oral health reduced the progression or occurrence of pneumonia. 1) There is fair evidence (II-2, grade B recommendation) of an association of pneumonia with oral health (odds ratio [OR]=1.2 to 9.6 depending on oral health indicators). 2) There is poor evidence of a weak association (OR<2.0) between COPD and oral health (II-2/3, grade C recommendation). 3) There is good evidence (I, grade A recommendation) that improved oral hygiene and frequent professional oral health care reduces the progression or occurrence of respiratory diseases among high-risk elderly adults living in nursing homes and especially those in intensive care units (ICUs) (number needed to treat [NNT]=2 to 16; relative risk reduction [RRR]=34% to 83%).