An exaggerated postprandial lipaemic response has long been implicated as a risk factor for coronary heart disease (CHD). The extent and duration of this postprandial response is regulated by lipoprotein lipase (LPL). It is also known that the fatty-acid composition of the diet can affect LPL activity. In this study, we therefore hypothesised that chronic dietary intakes of n-3 polyunsaturated fatty acids (n-3 PUFA) and monounsaturated fatty acids (MUFA) would modulate LPL activity, possibly by influencing gene expression. The objective of the 2 dietary intervention studies performed was to determine the feasibility of increasing dietary n-3 PUFA and MUFA intakes using fatty acid-modified, commercially produced foods in a free-living environment. The first study was a 3-week trial in which 9 middle-aged male volunteers consumed a variety of eicosapentaenoic acid (EPA)- and docosahexaenoic acid (DHA)-enriched foods. The second study was a 2-month trial in which 43 male volunteers increased their dietary MUFA (oleic acid) intake by replacing saturates, using olive oil-enriched foods. Significant increases in EPA+DHA (P < 0. 001) and oleic-acid (P < 0. 001) intakes were observed and the quality of the foods was excellent. These findings support the use of fatty acid-modified foods as a suitable vehicle through which to achieve changes in dietary composition. Few effects on postprandial lipid metabolism were found as a result of altering intakes of n-3 PUFA or MUFA. This suggests that adaptive responses to modified diets develop over a longer period of time than was investigated here. However, a significant reduction in plasma total cholesterol (P < 0. 02) and LDL cholesterol (P < 0. 001) concentrations was observed following substitution of MUFA for saturated fatty acids. Therefore, MUFA-enriched diets are a suitable alternative to low-fat, high-carbohydrate diets as a means of reducing plasma cholesterol levels and CHD risk. There is strong evidence that dietary fatty acids affect adipose-tissue LPL activity; however, to date it is unclear whether these effects are substrate or hormone driven. To investigate if the effect is substrate specific, a sensitive competitive RT-PCR method was developed. Although no significant changes in LPL gene expression or LPL activity were observed, there was a trend for the gene expression values to be higher following the n-3 PUFA and MUFA diets.