We examined how supplementation with very-long-chain n-3 fatty acids was reflected in the concentration of these fatty acids in plasma phospholipids of 363 Norwegian men and women. The concentration of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in plasma phospholipids was significantly higher among individuals supplemented with n-3 fatty acids after the supplementation period than before. We also examined the relation between dietary intake of fatty acids measured with a 180-item quantitative food-frequency questionnaire and the concentration of the same fatty acids in plasma phospholipids in 579 men and women. Correlation coefficients between plasma phospholipid fatty acids and dietary intake of fatty acids were 0.51 and 0.49 for EPA and DHA, respectively. The correlation between fish intake and n-3 fatty acids in plasma phospholipids was 0.37. These results suggest that dietary intake measured with our food-frequency questionnaire may be used to predict the biological availability of some of the essential n-3 fatty acids.