Abstract This study evaluates the influence of sex on platelet fatty acid (FA) composition, and whether sex differences are conditioned by age. Since plasma FA have a specific relationship with platelet FA their variations with age and sex are also considered. Forty-nine male-female human couples (16–75 years), where within each couple the partners were on qualitatively similar diets and of similar age, were studied. Few differences were found between the whole groups of men and women in platelet FA. A comparison of data on FA in platelet phospholipids (PL) from 3 age groups (16–40, 40–60 and over 60) showed an increase in saturated FA of middle-aged subjects, an age-dependent decrease in 20:5 in both sexes and of 18:2 mainly in women. The percentage of plasma phosphatidylserine plus phosphatidylinositol decreased in middle-aged subjects. With regard to the influence of FA of plasma PL on FA of platelet PL, we found a higher correlation coefficient ( r) for 16:0 and 18:0 and 20:4 and a lower one for 20:5 in middle-aged men and post-menopausal women. Considering that an increase in saturated FA and 20:4 and a decrease in 20:5 in platelet PL may increase platelet function, the plasma FA influence on platelets may help to explain the higher incidence of CHD in those groups of subjects.