Lipid and lipoprotein concentrations were studied in 12 hypothyroid and 11 hyperthyroid female subjects, both before and after therapy, and in 27 age matched female controls. Recognized clinical and laboratory criteria established the diagnosis. Lipoproteins, including the sub-fractions of the high density lipoproteins (HDL), were isolated by preparative ultracentrifugation, and the cholesterol (c) and protein (p) contents of each were determined. Total cholesterol, and in particular HDL-c, were elevated in the hypothyroid patients. The low density lipoprotein (LDL) -c/HDL-c ratio was 1.9 in this group, compared to 2.2 in the control group and 1.35 in the hyperthyroid patients. The HDL-2/HDL-3 ratio in the hypothyroid group was 3.75, as compared to 1.75 in the controls and 4.2 in the hyperthyroid group. Plasma triglycerides were moderately elevated in the hypothyroid patients and were significantly reduced in the hyperthyroid group. Total cholesterol was significantly lower in the hyperthyroid group as compared to the control group. Very low density (VLDL) cholesterol and protein were significantly increased and LDL and HDL cholesterol were reduced in the hyperthyroid patients. On rendering the patients euthyroid, most of these changes were reversed. Thyroid function profoundly affects lipoprotein concentration and composition. The change in the plasma HDL concentrations of the hypothyroid group questions the relationship of this group to arteriosclerosis. Therapy partially corrects the abnormalities, but complete correction may be related to duration of therapy.