Characterisation of long-term adherence to EPA and DHA intakes through biomarkers and dietary assessments has implications for interpreting the findings of long-term intervention studies. Adherence to dietary advice targeting an EPA+DHA intake of 1 g/d was examined over 1 year. Men and women (n 45) received dietary advice to increase EPA and DHA intakes from seafood, nutraceutical (fish oil) or functional food sources, while a fourth group received combined advice. Blood biomarkers and dietary intakes of EPA and DHA were evaluated at baseline and post-intervention at weeks 4, 8, 12, 24 and 52. Assessment by 3 d diet records indicated that EPA+DHA intakes increased relative to baseline in weeks 4–52 following the seafood, nutraceutical and combined advice (advice group × time effect, P= 0·03). The percentage of DHA in plasma and whole blood and the percentage of EPA in erythrocytes, plasma and whole blood were higher in weeks 4–52 when compared with the corresponding baseline measurement. In contrast, the percentage of DHA in erythrocytes increased to a maximum at week 12 and returned to baseline levels in weeks 24 and 52 (time effect, P< 0·01). Measurement of the percentage of DHA in erythrocytes indicates that adherence was sustained during the first 12 weeks following the dietary advice, while other blood measurements of the percentage of EPA and DHA and dietary assessment suggest short-term increases in EPA+DHA intakes immediately before weeks 24 and 52. The percentage of DHA in erythrocytes characterises adherence to EPA and DHA intakes in long-term interventions.