This pilot study of ethyl-eicosapentaenoate (ethyl-EPA) in the treatment of Alzheimer's disease aimed to estimate the magnitude of any change in measures of cognition during a 12-week treatment period. A simple linear design was used in which each patient had a baseline period of 12 weeks without treatment, followed by 12 weeks' treatment with ethyl-EPA. Blood samples were taken both before and after the treatment period to measure erythrocyte membrane fatty acids. Assessments comprised cognitive measures and visual analogue ratings of overall assessment of functioning. There was little difference between treatment and baseline periods in the rate of decline of efficacy measures, except for a small improvement in carer's visual analogue rating (P=0.02). It was concluded that it is unlikely there were any clinically important treatment effects of ethyl-EPA on cognition during the 12-week treatment period. A longer treatment period may be necessary to demonstrate efficacy of ethyl-EPA in this disorder.