BACKGROUND: Accruing evidence from observational and epidemiological studies suggests an inverse relationship between dietary intake of omega 3 polyunsaturated fatty acid (PUFA) and risk of dementia. Postulated mechanisms that might qualify omega 3 PUFA as an interventional target for the primary prevention of dementia include its anti-atherogenic, anti-inflammatory, anti-oxidant, anti-amyloid and neuroprotective properties. OBJECTIVES: To review the evidence that omega 3 PUFA supplementation prevents cognitive impairment and dementia in cognitively intact elderly persons. SEARCH STRATEGY: The Cochrane Dementia and Cognitive Improvement Group's (CDCIG) Specialized register, MEDLINE, EMBASE,CINAHL PsycINFO, AMED AND CENTRAL and several ongoing trials databases were searched on 5 and 6 October 2005. The CDCIG Register is updated regularly and contains records from all major medical databases and many ongoing trials databases. SELECTION CRITERIA: In order to be selected, trials needed to be randomized, placebo-controlled, doubled blinded, of minimum study duration of 6 months, involved persons aged 60 years and above without pre-existing dementia at study onset, and employed cognitive endpoints. DATA COLLECTION AND ANALYSIS: Reviewers, working independently, were to select, quality assess and extract relevant data where appropriate and possible. In comparing intervention with placebo, the pooled odds ratios or weighted mean differences and standardized mean difference were to be estimated. MAIN RESULTS: There were no randomized trials found in the search that met the selection criteria. Results of two clinical trials are expected in 2008. AUTHORS' CONCLUSIONS: There is a growing body of evidence from biological, observational and epidemiological studies that suggests a protective effect of omega 3 PUFA against dementia. However, until data from randomized trials become available for analysis, there is no good evidence to support the use of dietary or supplemental omega 3 PUFA for the prevention of cognitive impairment or dementia.