A prospective study of food intake using 7 day food diaries was undertaken in 92 diabetic men and women aged 17-81 years. The median individual day-to-day coefficients of variation for energy intake were: in insulin treated patients 12.0%, in non-insulin treated patients 13.7%; for carbohydrate intake 14.5% and 13.8% and for fat 20.7% and 20.8%, respectively. The median individual differences between the minimum and maximum daily intake of energy in insulin treated patients was 787 kcal, in non-insulin treated patients 649 kcal, for carbohydrate intake 89g and 77g and fat 50g and 43g, respectively. Only 39% patients ate within 20% of their prescribed carbohydrate diet. In non-insulin treated patients on prescribed calorie controlled diets, calorie consumption was on average 46% in excess of that prescribed. Although the variation in dietary intake in diabetic patients is large, it is smaller than that reported in non-diabetic subjects in the UK. This variation is likely to make the manipulation of other antidiabetic therapy both difficult and somewhat arbitrary.