With a view to clarify the cause of hypoplastic anemia the author studied the adrenal cortical function in patients with this disease, and obtained the following results. 1. The basic metabolic rates show not any definite inclinations. 2. In the insulin tolerance test, the insulin index has been found to have declined. 3. As for electrolytes in serum, chlorine is decreased but phosphorus is increased to a high degree, and ratio Na/K is slightly high while calcium, sodium and potassium show no definite inclinations. 4. Robinson-Power-Kepler water test is almost positive. 5. On measuring 17-KS and free chemocortioids (Ch. C.) excreted in the urine, the amount of 17-KS is decreased conspicuously but Ch. C. is increased. 6. In the gel-ACTH loading test the adrenal cortical reserve function has been found to have declined highly. 7. From these facts stated above, it is possible to assume that there is a decline in the adrenal cortical function in this disease.