The effects on plasma lipoproteins of four fat-modified diets were assessed in 11 nuns in a contemplative order in the Mediterranean region of Spain. Diet 1 [high polyunsaturated fatty acid (PUFA), low monounsaturated fatty acid (MUFA), low ratio of PUFAs to saturated fatty acids (P:S)] and diet 3 (low PUFA, high MUFA, low P:S) induced significant, directly comparable reductions in total plasma (12% and 13%, respectively) and low-density-lipoprotein (LDL) cholesterol (24% and 19%, respectively). Diet 2 [high PUFA, high MUFA, low saturated fatty acid (SFA), high P:S] induced greater decrements (23% and 30% in total plasma and LDL cholesterol, respectively). Diet 4 (low PUFA, low MUFA, high SFA, low P:S) induced a significant increase in LDL cholesterol of 11%. No significant changes in high-density-lipoprotein cholesterol were observed with these diets. Because the effects of PUFAs and MUFAs are comparable, no recommendations on modifying the habitual, high-MUFA-containing Mediterranean diet need be made other than, perhaps, a reduction in the overall intake of SFAs.