To determine the effects of a vegetarian diet with avocado as a source of monounsaturated fat on serum lipids, thirteen patients with phenotype II (twelve with IIa and one with IIb) dyslipidemia were included in a prospective, transversal and comparative study in which three four-week diets randomly assigned were assessed. One vegetarian diet (ALVD) was composed of 70% carbohydrates, 10% proteins and 20% lipids. Another was composed of 60% carbohydrates, 10% proteins and 30% lipids, 75% of which was supplied by avocado (AVD). A third diet was an avocado-added free diet (FDWA). Body weight, body mass index (BMI), and serum lipids (total cholesterol (TC), high (HDL) and low density lipoprotein (LDL) cholesterol and triglycerides (TG)) were evaluated. AVD produced a significant decrease in LDL. ALVD did not change TC and LDL, while FDWA increased them slightly. The three diets reduced TG levels, but only ALVD did so significantly. All three diets reduced HDL levels, particularly ALVD, which produced the greatest reduction. Low-fat, carbohydrate-rich vegetarian diets may be harmful to hypercholesterolemic patients. The avocado addition to a vegetarian diet does not correct these undesirable effects. To obtain beneficial effects on lipid profile with avocado, lower amounts of carbohydrates and polyunsaturated fatty acids are probably needed.