This study was conducted to examine the effect of ω3 fatty acid supplementation on plasma lipid, cholesterol and lipoprotein fatty acid content of non-insulin-dependent diabetic individuals consuming a higher (0.65, n = 10) or lower (0.44, n = 18) ratio of dietary polyunsaturated to saturated fatty acid (P/S). The participants were initially given an olive oil supplement (placebo) equivalent to 35 mg of 18:1 · kg body weight^–1· day^–1 for 3 months. This was followed by two ω3 supplement periods in a randomized crossover. In these 3-month periods, participants were given a linseed oil supplement equivalent to 35 mg of 18:3ω3 · kg body weight^–1· day^–1 or a fish oil supplement equivalent to 35 mg of 20:5ω3 + 22:6ω3 · kg body weight^–1· day^–1. At the end of each supplement period, a blood sample was drawn from each participant for lipid, lipoprotein, insulin, glucagon and C-peptide analyses. At the end of each 3-month period a 7-day dietary record was completed to calculate dietary fat intake and P/S ratio. Results indicate that fish oil significantly reduced plasma triacylglycerol level (p < 0.05) and increased 20:5ω3 and 22:6ω3 content of all lipoprotein lipid classes. Linolenic acid supplementation had no effect on plasma triacylglycerol level, but it increased 18:3ω3 content of lipoprotein cholesterol ester fractions (p < 0.05). A slight increase in 20:5ω3, but not 22:6ω3, content was noted in lipoprotein lipid classes as a result of 18:3ω3 supplementation. LDL and HDL cholesterol, insulin, glucagon and C-peptide levels were not affected by either ω3 supplement. It is concluded that a modest intake of ω3 fatty acids, such as could be obtained from consuming fish regularly, will reduce plasma triglyceride level without affecting LDL or HDL cholesterol levels. [Diabetologia (1997) 40: 45–52]