Although studies in those who have already experienced clinical episodes of ischaemic heart disease (IHD) have suggested properties of platelets influencing recurrence, there is limited information on the value of platelet tests in predicting first episodes of IHD. One study has suggested that a raised platelet count and increased aggregability in response to adenosine diphosphate (ADP) may increase IHD incidence but the numbers of IHD events involved were small. The larger Northwick Park Heart Study (NPHS) included platelet count and both ADP and adrenaline-induced aggregation and this paper presents their associations with subsequent IHD. Platelet counts were performed in 1369 white NPHS men aged between 40 and 64 at recruitment, of whom 181 subsequently experienced a major episode of IHD over a follow-up period of 16.1 years. Platelet count was unrelated to the incidence of IHD. ADP-induced aggregation was performed in a random sample of 740 men in whom 66 IHD events occurred during the subsequent 10.1 years, aggregability being measured both as ED50, the ADP dose at which aggregation occurred at half its maximum velocity, and also as EMR, the maximum rate of aggregation achieved. Neither measurement showed any association with IHD incidence, nor did similar measurements in 460 men in whom adrenaline-induced aggregation was also carried out. There are at least three possible explanations for the lack of any association between the measures of aggregability used and IHD. First, the large within-person variability of platelet aggregation tests may make the demonstration of any associations difficult, though the study had reasonable power to show effects with ADP. Secondly, the tests used may not be a valid index of the contribution of platelet function to thrombosis and IHD. However, the clear effect of several personal and demographic influences associated with IHD on the tests used brings this explanation into question. Thirdly, the role of platelets in thrombogenesis may be determined mainly by plasma influences such as fibrinogen, rather than by intrinsic properties of platelets themselves. Platelet counts within the physiological range and the aggregation tests used in this and in some other studies are of no value as indices of the risk of first episodes of IHD.