Goal: Systematic review of omega-3-fatty acids (ω3-FA) in dementia and analysis of handling missing participant data in clinical trials. Methods: Randomized controlled trials with ω3-FA in dementia were analysed according to the Cochrane methods. The handling of missing data in clinical trials on dietary supplements in dementia was explored using extended CONSORT criteria related to missing participant data. Results: In meta-analyzes, ω3-FA showed no benefit in Alzheimer's dementia in the primary outcomes cognition, everyday function, and severity of dementia. Missing participant data are not sufficiently reported. The impact of missing data was not investigated in three of the six included trials and the the potential for bias is not discussed in five publications. Conclusions: Ω3-FAs are not effective in Alzheimer's dementia. Missing participant data should be better investigated, reported and critically discussed.