Frangou et al (Reference Frangou, Lewis and McCrone2006) reported ethyleicosapentaenoic acid (ethyl-EPA) to be effective in the treatment of bipolar depression. However, no mention is made of the dietary intake of ethyl-EPA among the randomised groups. Ethyl-EPA is a naturally occurring substance and hence a potential confounding variable. The statistically significant improvements in the ethyl-EPA group(s) compared with placebo in terms of decreases in scores on the Hamilton Rating Scale for Depression and the Clinical Global Impression Scale could simply have been a result of differences in dietary ethyl-EPA intake. Such a difference is likely to have involved increased intake in the ethyl-EPA group(s), but decreased intake in these groups could have lessened any potential improvements. Further studies of omega-3 fatty acids must control for the potential confounding independent variable of dietary intake.