The extended use of diet and cholestyramine therapy in familial type II hyperlipoproteinemia was examined in patients who previously participated in a short-term, double-blind trial. A striking secondary failure in therapeutic response during 4 yr of use of this therapy was noted with plasma cholesterol rising an average of 15%. A 3 mo, out-patient, follow-up study designed to reinforce patient motivation and dietary and drug adherence resulted in a prompt but partial reversal of this therapeutic deterioration in 16 patients. Additional inpatient studies confirmed that patient noncompliance with the dietary regimen was the major factor responsible for the secondary failure. Cholestyramine together with a low cholesterol diet can be an effective agent in familial type II hyperlipoproteinemia, given a comprehensive program of out-patient follow-up with continued emphasis on dietary principles and drug adherence.