Twenty seven insulin-dependent (type I) diabetic patients (mean +/- SD; age: 30 y +/- 12; duration of diabetes 12 y +/- 2.5) were treated for at least 12 weeks with two regimens differing only in their fatty acid composition. The energy in the diabetic diet was contributed 50%-55% by carbohydrates, 15%-20% proteins and 30% by lipids. The diet high in poly-unsaturated-fatty-acids (PUFA) contained 61% PUFA and 17% of saturated fatty acids (SFA) while the diet low in PUFA was composed of only 4% PUFA and of 65% SFA. Group I followed the diet high in SFA for six weeks and then switched to the diet high in PUFA. Group II followed the diets in the reverse order. After six weeks of treatment with the high PUFA diet, total serum cholesterol decreased from 195 mg % +/- 11.1 mg % in group I and 179 mg% +/- 12.1 % in group II to 169 mg +/- 10.2 and 142 mg % +/- 8.1 respectively. This fall was mainly due to highly significant decrease in LDL-Cholesterol. VLDL-cholesterol and HDL-cholesterol in both groups remained almost unchanged. In group I a significant decrease in serum triglyceride from 88 mg % +/- 9.2 to 70 mg % +/- 10.9 was found after six weeks of treatment with the high PUFA diet. The atherogenic index decreased significantly in both groups. Six weeks of treatment with the high SFA diet reversed these results. Serum cholesterol increased mainly due to the highly significant increase in LDL-cholesterol, VLDL-cholesterol, HDL-cholesterol and triglycerides remaining unchanged.(ABSTRACT TRUNCATED AT 250 WORDS)