The metabolic effects of fructose incorporated in the normal diets of 13 poorly controlled, Type 2 diabetic patients were studied in a 6-month, randomized, crossover study. Patients used 60 g day-1 of crystalline fructose in divided amounts as part of their isocaloric (1400-3900 kcal), weight-maintaining diet. During fructose supplementation, the distribution of carbohydrate-derived calories was 35% complex and 15% simple, the latter solely from fructose. This was compared with the patients' values on their usual diabetic diet (carbohydrate 50% (mostly complex), fat 38%, and protein 12%). After 6 months of taking fructose, fasting serum glucose decreased from 12.6 +/- 1.1 (+/- SE) to 9.8 +/- 1.3 mmol l-1 (p less than 0.02), while it was unchanged on normal diet (11.0 +/- 0.1 vs 11.6 +/- 0.9 mmol l-1, NS). Glycosylated haemoglobin was also reduced from 11.3 +/- 0.4 to 9.9 +/- 0.5% (p less than 0.05) on fructose, but unchanged on the control diet (10.4 +/- 0.7 vs 11.2 +/- 0.7%, NS). No significant long-term deleterious changes were observed in the fasting serum lipids, lipoproteins, and apolipoproteins A-1 and B-100. Fructose was well tolerated without significant effects on body weight, or lactic acid and uric acid levels.