Effects of cis and trans monounsaturated fatty acids (TFA) and saturated fatty acids were assessed in 29 men and 29 women consuming controlled diets. Subjects ate each diet for 6 wk in a Latin square design. The diets, each with 39-40% of energy as fat were: 1) high oleic (16.7% of energy as oleic acid), 2) moderate TFA (3.8% of energy as TFA), 3) high TFA (6.6% of energy as TFA), 4) and saturated (16.2% of energy as lauric+myristic+palmitic acids). Compared with the oleic diet, LDL cholesterol increased 6.0%, 7.8%, and 9.0% after moderate TFA, high TFA, and saturated diets, respectively. HDL cholesterol was unchanged after moderate TFA, but was slightly lower (2.8%) after high TFA. HDL cholesterol after the saturated diet was 3.5% higher than after the oleic diet. Changes in apolipoproteins B and A-I corresponded with changes in the lipoprotein cholesterols. Thus, compared with oleic acid, dietary TFAs raise LDL cholesterol, but to a slightly lesser degree than do saturates, and high TFA concentrations may result in minor reductions of HDL cholesterol.