The intake of a stable diet during 25 months by 68 women living in a closed religious community allowed us to study the effects of various alimentary fats on the serum lipids. Thus we could observe the consequences of the quantitatively and qualitatively different fatty acids in the diet, while the total quantity of lipids and the other constituents of the dietary intake were unaltered. The serum total cholesterol and apoprotein B levels are correlated with the P/S ratio of the diet; on the contrary, the serum HDL cholesterol and apoprotein A levels are not correlated with the fatty acid composition of the diet. The moderate supply in lipids (30% of the total caloric intake) and cholesterol (300 mg/day) maintained during this study corresponds to the usual dietary recommendations. Therefore, not very high levels of serum total cholesterol were observed, whichever vegetable oil was consumed.