Low serum cholesterol levels have been reported to occur in patients with macrocytic anemia due to folic acid or vitamin B12 deficiency or both. Dietary fat intake did not appear to be in any way related to the serum cholesterol levels. The lipoprotein distribution pattern was determined in few patients, and the result showed the β-lipoprotein levels to be decreased in all fractions, with the most significant depression in the Sf 0-12 fraction, which carries most of the serum cholesterol. A definite statistical correlation between serum cholesterol levels and serum folate levels up to 6.2 mug/ml has been established. Significantly lower correlation was obtained for vitamin B12 deficiency, even though in all patients with very severe vitamin B12 deficiency, serum cholesterol patterns resembling those of folate deficiency have been observed. A return of serum cholesterol to normal levels has been observed in all patients after folate administration.