OBJECTIVE: Two experimental diets were evaluated to investigate the hypothesis that dietary fat has an independent influence on metabolic control in IDDM. RESEARCH DESIGN AND METHODS: The diets had similar CHO contents (26 and 22% of energy intake) but differed markedly in fat (53 vs. 16% energy) and protein (20 vs. 62% energy). We had 10 subjects follow the low-CHO, high-fat diet, and 8 subjects follow the low-CHO, low-fat, high-protein diet. In each case, markers of glycemic and lipid control obtained after adherence to the experimental diet for 2 wk were compared with corresponding data from a preceding control period during which subjects had followed their usual diet (protein 18-19%, CHO 41-46%, fat 33-37%). RESULTS: Despite the low CHO content of the high-fat diet, insulin requirements were unchanged relative to the control diet. Moreover, the glycemic response to a standard breakfast was elevated significantly (P < 0.001), suggesting that insulin resistance had either been induced or exacerbated. The small rise in total cholesterol concentration in response to the high-fat diet was accounted for by a rise in HDL cholesterol. Glycemic control and lipid metabolism were unchanged after the low-CHO, low-fat diet, although insulin requirements fell by an average of 6 U/day (P < 0.05) relative to those recorded during the 2-wk control period. CONCLUSIONS: Diets high in fat are deleterious to glycemic control in IDDM, but general applicability is limited by the small sample size and short duration of this study.