The effectiveness of a new, almost l-thyroxine free preparation of d-thyroxine (Dynothel) was tested in 15 patients with Type IIa and 4 patients with Type IIb hyperlipoproteinemia. Eleven patients with Type IIa and 3 with Type IIb were responsive to treatment and showed an average 26% decrease in plasma TC. This decrement in plasma TC was mirrored in a significant reduction of LDL cholesterol in Type IIa and IIb. While VLDL cholesterol slightly decrease in Type IIb, it remained the same in Type IIa and so did the HDL cholesterol in both types. As neither VLDL nor LDL or HDL triglyceride levels changed very much in either type, the total plasma triglycerides remained the same. The plasma phospholipids were higher in Type IIa and lower in Type IIb on therapy. Thus, Dynothel seems to be a potent d-thyroxine preparation for lowering plasma cholesterol, this decrease being brought about by reduction of LDL cholesterol levels. The effect of the drug on plasma TG and PL is less certain.