Diet was measured by 5-day weighed inventory to search for an association between fat intake in the general population and factor VII coagulant activity (VIIc), a strong predictor of coronary heart disease. Of 275 men aged 40-59 years registered with a medical practice, 203 (74%) participated and 170 (62%) provided a satisfactory record. After allowance for the increase in fat intake with body size, a statistically significant and positive association was found between dietary fat and VIIc (r = 0.18; P less than 0.05). The correlation coefficient was increased to 0.24 when adjusted for the effect of day-to-day variability in individual fat intake, thereby providing an improved estimate of the true strength of association. The mean difference in VIIc of 12% of standard between men in the highest and lowest quarters of the distribution of fat intake was similar to that reported between men experiencing coronary heart disease and those remaining free. The results support previous experimental fat-feeding studies and suggest that a high fat diet has adverse consequences for blood coagulability and coronary thrombosis.