Ten diabetic patients were given test meals at breakfast and lunch on successive days to compare the acute glycaemic responses to meals either high (65%) (HC) or extremely low (12%) in carbohydrate; the latter was very high (71%) in fat (HF). The meals were isocaloric and fibre content, though higher in HC, was within the range of the average British intake for both diets. The mean fasting blood glucose levels were similar on the 2 days (9.8 mmol/l; 9.6 mmol/l) but mean peak post-prandial levels were significantly greater after the HC meals (17.7 mmol/l after breakfast and 14.1 mmol/l after lunch) than after the HF meals (11.7 mmol/litre after breakfast; P less than 0.02; 8.1 mmol/l after lunch; P less than 0.02). Mean blood glucose value for the 6-hr study period was significantly higher after the HC meals (14.0 mmol/l) than after the HF meals (9.2 mmol/l; P less than 0.001). There was no significant difference in mean fasting, peak or total mean triglyceride level. In the short term, therefore, meals very low in carbohydrate and high in fat are followed by a lower glycaemic response than meals high in carbohydrate but with only a usual (22 g/day) fibre content. Delayed gastric emptying after HF meals may well contribute to the results but long-term use of such HF meals is not advised in view of the extremely high fat content.