This communication describes an investigation of the effectiveness of dietary counseling in the treatment of patients classified phenotypically as having type IV hyperlipoproteinemia or endogenous hypertriglyceridemia. Over a period of two years, 103 patients who received instructions for a therapeutic diet and regular dietary counseling by a physician and dietitian had a large decline in the concentration of serum triglycerides, as compared to 175 patients with the same diagnosis who received similar diets but little dietary counseling. Over the period of the study, the median plasma triglyceride concentration decreased from 302 to 210 mg/100 ml, the mean triglyceride value from 381 to 263 mg/100 ml, and the mean weight from 84 to 80 kg in the group receiving dietary counseling.