Evidence assembled here indicates that when olive oil forms a major part of dietary fat in controlled human experiments, total and LDL-cholesterols are somewhat higher than when the same amount of fat is one of the modern predominantly monounsaturated oils: low erucic rapeseed or high oleic sunflower oil. Oils rich in monounsaturated fatty acids thus do not all have the same effect on plasma cholesterol. This phenomenon is explicable by consideration of the content of other fatty acids and the non-saponifiable fractions of the different monounsaturated oils. It helps to explain the discrepancy that has existed between the classic experiments (using olive oil), which found monounsaturated oils 'neutral', and some of the more recent experiments which found them more cholesterol-lowering than carbohydrates. Four published meta-analyses are reviewed. The three which included most of the published experiments show that monounsaturated fatty acids (MUFA) have less plasma cholesterol-lowering effect than polyunsaturated fatty acids.