In the present intervention study we compared the effects of alpha-linolenic acid with those of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) on platelet aggregation in vitro and ex vivo in healthy non-institutionalized elderly subjects. We also compared the effects of alpha-linolenic acid on platelet aggregation in elderly subjects with those in younger volunteers. During a run-in period of 3 weeks all subjects (thirty-eight elderly (> 60 years) and twelve younger volunteers (< 35 years)) received a diet rich in oleic acid. For the next 6 weeks the elderly subjects received a diet rich in oleic acid (n 11), alpha-linolenic acid (n 14) (6.8 g/d) or EPA/DHA (n 13) (1.05 g EPA plus 0.55 g DHA). The younger subjects were given a diet rich in alpha-linolenic acid. The diets did not affect ADP- or collagen-induced platelet aggregation in vitro in either platelet-rich plasma or whole blood. The ex vivo platelet aggregation as measured with filtragometry was significantly decreased in the elderly group that received EPA/DHA compared with the alpha-linolenic acid (P = 0.006) and the oleic acid (P = 0.005) diet groups. Effects of alpha-linolenic acid were not age-dependent. Our results suggest that alpha-linolenic acid and EPA/DHA do not change in vitro platelet aggregation. Compared with oleic acid, EPA/DHA, but not alpha-linolenic acid, favourably affects ex vivo platelet aggregation in healthy elderly subjects.