A lipid intervention clinic screened 4000 employees (89% participation) and identified 150 type IV subjects (top 5 percentile triglyceride values, 100% initial participation, 6% drop out). The 150 healthy type IV subjects, ages 20 to 49, were randomly divided into treatment subgroups: A, treatment by clinic nutritionist and physician with the National Heart and Lung Institute's type IV diet for 6 weeks, then diet plus clofibrate for 18 weeks; B, same treatment by private physician; C, no intervention for 24 weeks, subjects advised of abnormality. The group A mean fasting serum triglyceride of 407 mg/dl declined 50% at 6 weeks, 61% at 12 weeks, and was unchanged at 24 weeks (p<0.0005 at 6, 12, 24 weeks). Group B triglyceride decreased 42%, 50%, 41% (p<0.0005) at 6, 12, 24 weeks). Group C triglyceride declined 20%, 1st to 24th week. Body weight decreased 8% (A) and 4% (B) at 6 weeks (p<0.0005) and was unchanged at 24 weeks. The maximum cholesterol decrease (A) was 11% (p<0.0005). Type IV hyperlipoproteinemia can readily be identified in a working population; treatment by clinic or private physician will markedly lower fasting serum triglyceride values in apparently healthy type IV subjects for at least 24 weeks.